A New Bromo Trienyne: Synthesis of all-E, Conjugated Tetra-, Penta-, and Hexaenes Common to Oxo Polyene Macrolide Antibiotics.

In a recent report from these laboratories,1 a new “linchpin” 1 was disclosed that allows for rapid construction of the all-E oxopolyene network characteristic of many polyene macrolide antifungal agents (Figure 1).2 This methodology relies on an initial Pd(0) coupling, where 1 serves as the nucleophilic partner. The acetylenic terminus can be regioand stereoselectively hydrozirconated, and while introduction of an acyl moiety could be accomplished in the presence of Me2AlCl, a second Pd(0)-catalyzed vinyl-vinyl coupling was not realized due to the highly deactivated, conjugated vinylic zirconocene.4 This limitation encouraged us to pursue a second-generation reagent that would make available not only all-E oxo tetraand oxo pentaenes but also the oxo hexaene framework as well. We now describe a redesigned tetraene equivalent 2, which provides synthetic opportunities not available to 1. Bromo trienyne 2 is prepared via E-bromo dienal 4 and the ylide derived from 5 utilizing a standard Wittig protocol (Scheme 1). Known precursor potassium salt 3 (mp > 350 °C) is obtained from inexpensive pyridine‚sulfur trioxide complex.5 Conversion of 3 to bromo dienal 4,6 reported to proceed using Br2/PPh3 in CH2Cl2, in our hands affords low yields of desired product. Attempts to modify conditions (e.g., changing the solvent to 1,2-dichloroethane, adding Bu4NX, various concentrations, and temperatures) or conversion to other leaving groups (e.g., the triflate derivative of 3) were not productive. In time, we found that use of NBS/PPh3 led to a good isolated yield of 4 (74%; 68:32 E/Z, separable by chromatography). The corresponding iodide6 could likewise be prepared using NIS/PPh3 (76%; 1:1 E/Z). Treatment of phosphonium bromide 57 with NaN(TMS)2 in THF8 followed by aldehyde (E)-4 (mp 66-68 °C) affords tetraene equivalent 2 in 86% yield as an g85:15 mixture of E,E,E to E,E,Z isomers. The vinyl bromide portion of 2 represents a polarity inversion relative to stannyl dienyne 1 and, hence, could be coupled with vinyland dienylzinc reagents 6 (n ) 1, 2; Scheme 2). Nucleophilic partners appear to tolerate TIPSprotected alcohols, substituted styryl residues, and divalent sulfur (Table 1). Yields tend to be uniformly good, and the ratio of E:Z products associated with the newly formed bond reflects maintenance of stereochemical integrity, as expected.9 These initial products could be desilylated to 7 and either hydrozirconated and then transmetalated to aluminum with Me2AlCl or carboaluminated directly to the corresponding vinylalane 8.10 Subsequent exposure to a chloroformate (or acid chloride) affords the desired conjugated polyene esters 9 (or ketones). Representative examples are illustrated as well in Table 1. Particularly noteworthy cases include (1) the entire polyene section of the mycoticins11 (entry 2) and (2) the alarm pheromone navenone C (entry 4).12 The overall stereochemical outcome of these reactions, as noted previously,1 is such that essentially all-E products are obtained notwithstanding the g85:15 mix of polyenynes 7 formed from the vinyl-vinyl cross-coupling/desilylation. The enhancement results not from eventual isomerization but rather a kinetic resolution based on the greater reactivity of the Evs Z-vinylalane intermediate 8 toward the electrophile. * To whom correspondence should be addressed. Phone: (805) 893-2521. Fax: (805) 893-8265. E-mail: Lipshutz@chem.ucsb.edu. (1) Lipshutz, B. H.; Lindsley, C. J. Am. Chem. Soc. 1997, 119, 4555. (2) Rychnovsky, S. D. Chem. Rev. 1995, 95, 2021. Omura, S.; Tanaka, H. In Macrolide Antibiotics: Chemistry, Biology, and Practice; Omura, S., Ed.; Academic Press: New York, 1984; pp 351-404. (3) Carr, D.; Schwartz, J. J. Am. Chem. Soc. 1979, 101, 3521. (4) Negishi, E.; Owczarczyk, Z. Tetrahedron Lett. 1991, 46, 6683. (5) Becher, J. Org. Synth. 1979, 59, 79. (6) Soullez, D.; Ple, G.; Duhamel, L.; Duhamel, P. J. Chem. Soc., Chem. Commun. 1995, 563. For a very recent report describing an improved route to 4, see: Vicart, N.; Castet-Caillabet, D.; Ramondenc, Y.; Ple, G.; Duhamel, L. Synlett 1998, 411. (7) Corey, E. J.; Ruden, R. A. Tetrahedron Lett. 1973, 1495. (8) Reitz, A. B.; Nortey, S. O.; Jordan, A. D.; Mutter, M. S.; Maryanoff, B. E. J. Org. Chem. 1986, 51, 3302. (9) Hegedus, L. S. In Transiton Metals in the Synthesis of Complex Organic Molecules; University Science Books: Mill Valley, CA, 1994. Stille, J. K.; Groh, B. L. J. Am. Chem. Soc. 1987, 109, 813. (10) Okukado, N.; Negishi, E. Tetrahedron Lett. 1978, 2357. (11) Wasserman, H. H.; Van Verth, J. E.; McCaustland, D. J.; Borowitz, I. J.; Kamber, B. J. J. Am. Chem. Soc. 1967, 89, 1535. Poss, C. S.; Rychnovsky, S. D.; Schreiber, S. L. J. Am. Chem. Soc. 1993, 115, 3360. (12) Sleeper, H. L.; Fenical, W. J. Am. Chem. Soc. 1977, 99, 2367. Figure 1.