Lactone hydroboration-oxidation reaction

The rhodium-catalyzed hydroboration has opened the way to cyclization reactions starting from dienes [92], For instance, rhodium-catalyzed hydroboration of the terminal alkenyl group of an os/Tunsaturated lactone followed by reaction with the PTOC-OMe chain transfer reagent afforded the bicyclic a-S-pyridyl lactone in 63% yield (Scheme 39). After oxidation of the sulfide with m-CPBA, thermal elimination of the sulfoxide afforded the corresponding a-methylene lactone in 65% yield. Interestingly, such bicyclic a-methylenelactones are substructures that can be found in many natural products such as mirabolide [93]. 

The reduction of epoxides withborane is noteworthy as it gives rise to the less substituted alcohol as the major product (7.96), in contrast to reduction with complex hydrides (compare with Scheme 7.71). The reaction is catalysed by small amounts of sodium or lithium borohydride and high yields of the alcohol are obtained. With 1-alkylcycloalkene epoxides, the 2-alkylcycloalkanols produced are entirely cis, and this reaction thus complements the hydroboration-oxidation of cycloalkenes described in Section 5.1, which leads to trans products. Reaction with borane in the presence of boron trifluoride has also been used for the reduction of epoxides and for the conversion of lactones and some esters into ethers. 

At first, both terminal double bonds of 5 are hydroborated and the diol, obtained after oxidative work-up, is treated with mild acid (PPTs) resulting in the selective protection of one OH function through lactone formation. Now, the other OH group can be selectively oxidized to give the aldehyde 16, which is then converted in a Wittig reaction to the a,/(-unsaturated ester 17. The transformation of 17 to the dialdehyde... 

Oxidative hydroboration 91) of lactone 11.49 afforded a mixture of four lactarorufins (Scheme 20), in which diols 11.30 and 11.33, arising from a attack of diborane, largely predominated (more than 90%). The same type of stereoselectivity was observed for other addition reactions, i.e. epoxidation, osmylation, hydrogenation (775), (704), 43) to 2,9, 3,4- or 6,7-double bond of lactaranolides and marasmanes. Apparently, the tricyclic structures of these substrates provided enough conformational and steric bias to direct approach of reagents from the same side as the bridgehead protons H-2 and H-9. However, when the double bond was located in a different position, exceptions were observed 98). 

---