Pergamon Tetrahedron Letters 40 (1999) 37-40 TETRAHEDRON LETTERS THE REACTION OF a,~-ACETYLENIC KETONES WITH DICYCLOHEXYLBORANE: STEREOSELECTIVE SYNTHESIS OF FUNCTIONALIZED TRISUBSTITUTED OLEFINS George W. Kabalka,* Su Yu, Nan-Sheng Li, Ute Lipprandt Departments of Chemistry and Radiology The University of Tennessee Knoxville, TN 37996-1600; Email: Kabalka@utk.edu Received 15 September 1998; revised 13 October 1998; accepted 15 October 1998 Abstract: Allenoxyborinates, easily generated via the 1,4-addition of dicyclohexylborane to a,~- acetylenic ketones, react in situ with excess starting ketone to afford stereodefined, functionalized trisubstituted olefins in high yields. © 1998 Elsevier ScienceLtd. All rights reserved. The hydrobortion of functionalized alkenes and alkynes has played an important role in organic synthesis.1 Recently, we reported that the carbon-carbon double bonds and triple bonds of non- conjugated unsaturated ketones and aldehydes can be selectively hydroborated by dicyclohexylborane.2 However, reactions of a,13-unsaturated ketones and aldehydes with boron reagents generally proceed in a different manner. For example, Trombini and his coworkers report that conjugated olefinic ketones react stereoselectively with dicyclohexylborane or (-)- diisopinocampheylborane to form boron enolates, a The carbonyl group in a,13-olefinic ketones is simply reduced by 9-BBN. 4 Enantioenriched propargylic alcohols are obtained by reduction of a,13- acetylenic ketones using chiral boron reagents, such as 13-isopinocampheyl-9-BBN, s 13- chlorodiisopinocampheylborane 6 and BMS in the presence of chira113-methyl-4,5,5-triphenyl-l,3,2- oxazaborolidine.7 However, to the best of our knowledge, no report on the reactions of a, ~-acatylenic ketones with dialkylboranes has appeared in the literature. As noted in our earlier reports, 2 dicyclohexylborane reacts preferentially with the triple bond in non-conjugated alkynyl ketones to yield the hydroboration product, whereas 9-BBN is more reactive toward the carbonyl group. However, a,~-acetylenic ketones offer the possibility of a 1,4-addition reaction. 3-Heptyn-2-one was used as a model compound in our study. We verified that the reaction of a molar equivalent of 3-heptyn-2-one with 9-BBN produced 3-heptyn-2-ol in 95% yield but the reaction of a molar equivalent of 3-heptyn-2-one with dicyclohexylborane gave complex results. However, when two molar equivalents of 3-heptyn-2-one were added to a slurry of dicyclohexylborane at 0 °C, the only product isolated was 7-acetyl-6-hydroxy-6-methyl-7-undecen-4-yne(2a) (Scheme 1).s A variety of a,13-acetylenic ketones were then allowed to react with dicyclohexylborane. In each case, the aldol product was obtained as the major product (Table 1). 0040-4039/99/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved. Plh S0040-4039(98)02259-X