In 1995, isolation and structural characterization of two new 22-membered macrolides, dolabelides A (1) and B,¹ from the sea hare Dolabella auricularia were reported. Isolation of dolabelides C and D,² 24-membered macrolides, was achieved from the same source. Cytotoxicity studies of dolabelides A-D revealed promising results against cervical cancer HeLa-S₃ cells with IC₅₀ values of 6.3, 1.3, 1.9, and 1.5 g/mL, respectively. Synthetic studies toward various subunits of dolabelide have recently been reported.³ These efforts include synthesis of protected intermediates of the C1-C14 subunit, with Leighton and co-workers reporting the properly acetylated C1-C14 fragment and completing the only total synthesis of dolabelide D in 2006.⁴ Key features common among the dolabelide family are 11 stereogenic centers, eight of which bear oxygen, and two E-configured trisubstituted olefins. Other attributes possessed by this family of molecules include 1,3-anti-diol fragments found at C7/C9 and C19/C21, along with an accompanying 1,3-syn-diol at C9/C11 and polypropionate fragments at C1/C4 and C22/23. The endgame strategy for ring closure was envisioned to hinge on a key ring-closing metathesis (RCM) of the C14/C15-trisubstitued olefin following a precedent set by Leighton and co-workers.⁴ Preceding this macrocyclization is a simple esterification connecting the C1 carboxylic acid and C23 alcohol, thus coupling the C1-C14 (2) and C15-C30 (3) subunits of dolabelide. The stereochemical complexity and known biological activity of the dolabelide family present a worthy and formidable challenge. In this regard, we herein report the use of a multifaceted phosphate tether toward the synthesis of the C1-C14 subunit of dolabelide.

The cornerstone for the title work hinged on recent studies, countering historical views, which have revealed a functionally active phosphate triester tether⁵ within the P-chiral bicyclic phosphate 5 (Scheme 1).⁶ These studies also revealed selective cleavage pathways operative through displacement reactions at carbon (S_N2, S_N2') and phosphorus, ultimately affording multipositional activation, which extends throughout the bicyclic framework. Overall, rapid access to advanced polyol synthons was attained, thus providing impetus for this study.⁵

Scheme 1. Retrosynthetic Analysis of Dolabelide

Retrosynthetic analysis shows that assembly of the C1-C14 (2) portion can be achieved via a Grignard addition into the C11 aldehyde, which is accessed by regioselective hydride opening of the advanced phosphate intermediate 4. Regioselective cross metathesis (CM) between bicyclic phosphate (R,R)-5 and terminal olefin 6 generates the C5-C6 bond and installs five of the six stereocenters found within the C1-C14 subunit of dolabelide. Bicyclic phosphate tether (R,R)-5 is readily constructed from the proper enantiomeric, C₂-symmetric 1,3-anti-diol, (R,R)-7 via a P-tether-mediated diastereotopic differentiation using RCM. The C15-C30 portion of dolabelide can also be accessed using this phosphate methodology and the enantiomer of the C₂-symmetric 1,3-anti-diol (S,S)-7.

Synthesis of CM partner 11 was achieved through initial reduction of TBS-protected Roche ester 8, followed by subsequent Swern oxidation of the alcohol, providing the necessary aldehyde 9 (Scheme 2).⁷ Reaction of the formed aldehyde with the Z-crotyl (-)-Ipc-borane generated enantiopure homoallylic alcohol 10 in 80% yield.⁸ PMB-protection of alcohol 10 was achieved using p-methoxybenzyl-bromide and sodium hydride to afford 11 in 95% yield.⁹

	Scheme 2. Synthesis of CM Partner 2.3
	Scheme 3. CM Studies with Bicyclic Phosphate 5

A key component of the proposed synthesis of dolabelide was the selective CM between bicyclic phosphate (R,R)-5,^5a and the synthesized homoallylic alcohol 11. Previous studies have shown CM of bicyclic phosphate (R,R)-5¹⁰ in which the exocyclic olefin was shown to possess Type III olefin behavior, implying that no detrimental homodimerization pathways are operative.^10,11 Other derivatives of 11 (TBDPS (12)^12a and PMP acetal(13)^12b) were synthesized to test their ability to undergo the necessary CM reaction. Utilizing TBDPS as a protecting group (12) gave low reactivity, as well as low yields during CM (Scheme 3, entries 1-3). CM was next attempted between (R,R)-5 and 11 under the conditions previously reported (6 mol % Hoveyda-Grubbs cat., DCM, 45 C),¹⁰ but incomplete consumption of the starting phosphate was observed after 6 h (entry 4). Optimizing the reaction conditions with various solvents revealed that use of toluene (90 C), with the same catalyst loading, gave essentially the same results (entry 5). However, an improved yield of 60% was obtained (entry 6) when 12 mol % catalyst was added to the CM reaction. Switching to DCE (90 C) and adding only 6 mol % Hoveyda-Grubbs catalyst gave the optimized results for 11, providing 72% yield of CM product 14 after 2 h. When removing the silyl protecting group altogether, 13 furnished results similar to those with 11. It should be noted that in all cases excess 11, 12, 13, Type II CM partners, could be recovered in near quantitative yield and recycled in future CM events. This differential reactivity pattern toward CM demonstrates that both proximal and distal steric interactions play vital roles in the success of selective CM reactions.¹³

With optimal CM conditions, 11 was chosen over 13, owing to the facile removal of the silyl protecting group in the late stages of the synthesis. The CM between (R,R)-5 and 11 provided phosphate 14 in 72% yield on multigram scale (Scheme 4). Regioselective hydrogenation of exocyclic C5-C6 olefin in the presence of the C10-C11 internal olefin was paramount to allow for subsequent regioselective opening of the bicyclic system. Upon investigating several hydrogenation conditions, (Wilkinson's catalyst, Crabtree's catalyst, Pd/C) it was found that an in situ generated diimide reduction under mild conditions (o-nitrobenzenesulfonyl-hydrazine,¹⁴ Et₃N, DCM) provided the necessary regioselective hydrogenated phosphate 15 with near complete selectivity for the exocyclic olefin. Other diimide conditions (tosylhydrazine, NaOAc, H₂O, DCE, 90 C) gave drastically lower yields, likely due to bicyclic phosphate instability under basic medium.

Scheme 4. Selective Phosphate Cleavage in the Synthesis of Phosphate 17

With phosphate 15 in hand, efforts were directed to an additional use of the tether in a potential regioselective olefin transposition to the desired terminal olefin. Initial attempts focused on the use of allylic hydride addition employing various reagents (Stryker's reagent, CuCN·LiCl/PhSiH₃, CeCl₃·7H₂O/NaBH₄). To our dismay, however, all conditions probed provided only unreacted starting material or total decomposition of the reaction mixture. Pd-catalyzed formate reductions were next investigated for generation of the requisite terminal olefin.¹⁵ Employment of 1.5 equiv of formic acid and 5 mol % palladium acetate at 40 C in DCE selectively opened phosphate 15 to provide the desired terminal olefin. Methylation of phosphate acid intermediate showed that a highly regioselective process was operative (37:1 ratio of regioisomers as evident by ³¹P NMR analysis). Purification provided phosphate 17 in 87% yield. The remarkable regioselectivity reveals another feature of the phosphate tether, whereby orthogonal orbital alignment⁵ within 15 allows for selective Pd(0)-catalyzed allylic phosphate ionization at C12 over C9.

Scheme 5. Synthesis of the C1-C15 Subunit of Dolabelide

Installation of the C11-C14 fragment began with cleavage of the phosphate 17 using LiAlH₄, which generated a diol that was subsequently protected as the acetonide (PPTS, 2,2-methoxypropane, DCM) to yield 18 in 96% yield (Scheme 5). Ozonolysis (O₃, pyridine, DCM:MeOH 1:1, Me₂S) of the terminal olefin produced the intended aldehyde, which was subjected to the Grignard generated from 1-iodo-3-methyl-3-butene¹⁶ affording 19 in a 95% yield. Dess-Martin periodinane (DMP, NaHCO₃, DCM) oxidation of the free alcohol in 19 generated the requisite ketone in 90% yield. Attempts to selectively reduce the acetonide protected ketone, using an assortment of reducing agents, resulted in no diastereoselectivity at C11.¹⁷ This problem seemed likely to be circumvented by deprotection of the acetonide and subsequent syn reduction utilizing the C9 free alcohol. Removal of the acetonide was achieved by the addition of CeCl₃·7H₂O and water,¹⁸ which efficiently cleaved the acetonide protecting group without loss of the primary TBS group and provided diol 20 in 86% yield. Final chelation-controlled reduction of ketone 20 using Et₂BOMe and NaBH₄ afforded triol 21 in 60% (95% based on recovered starting material) with excellent diastereoselectivity (ds 20:1).

In conclusion, successful completion of the synthesis of the C1-C14 subunit of dolabelides A-D using phosphate tether methodology has been achieved. Overall, the phosphate tether serves a multifaceted role by (i) mediating the initial desymmetrization event leading to (R,R)-5, (ii) a selective Type III CM to couple two major complex fragments in the C1-C14 subunit of dolabelides, (iii) differentiating the two olefins within the bicyclic system 14, allowing for a selective hydrogenation, and finally (iv) serving as an excellent leaving group in a regioselective Pd(0)-mediated formate reduction. The route outlined above makes use of orthogonal protecting- and leaving group properties innate to phosphate esters. This work provides evidence that counters historical and traditional applications associated with the utilization of phosphates in synthesis. Efforts to complete the total synthesis are ongoing in our laboratories and will be reported in due course.

Acknowledgment

This investigation was supported by funds provided by NSF CHE-0503875, NIH RO1 GM077309, and the NIH Dynamic Aspects in Chemical Biology Training Grant (J.D.W.). We also thank Materia for supplying metathesis catalyst as well as Danielle Coverdell (University of Kansas) and John Nguyen (University of Kansas) for their synthetic contributions.