Aldehydes, reduction with Selectride

An excellent, broad review of the last 60 years of hydride reductions has been published,235 and the use of selectrides, Li and K tri-.v-butylborohydridcs or trisiamylborohydrides, has also been reviewed.236 A review of sodium borohydride-carboxylic acid as a reagent with novel selectivity in reductions has been written in particular, this reagent is useful for the A -alkylation of primary and secondary amines, through a sequence that is believed to involve sequential carboxylic acid to aldehyde reduction followed by reductive animation.237... 

Epoxidation of oxonine 93 with dimethyldioxirane, followed by reduction with diisobutylaluminium hydride (DIBAL-H), resulted in a separable mixture of alcohols 95 and 96, and the side product 94 (Scheme 16). Each of the isomers was submitted to Swern oxidation and sequential stereoselective reduction with L-selectride to achieve desired stereochemistry of the products 97 and 98. Formation of the side product 94 was explained by Lewis acidity of DIBAL-H and confirmed by treatment of oxirane derived from 93 with another Lewis acid, AlMe3, to produce oxocine aldehyde 99 in 35% isolated yield <1997CL665>. Similar oxidative synthetic sequence was utilized for the synthesis of functionalized oxonines as precursors of (-l-)-obtusenyne <1999JOG2616>. 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

The second has been used to make prostaglandins from the intermediate aldehyde 186. Note the stereochemistry the obvious trans arrangement of the two substituents in 184 after the Michael addition and the approach of the bulky reducing agent L-selectride Li(s-Bu3BH) from the opposite face to the nearer and larger of the two substituents. Finally, a reductive workup (Me2S) is needed after the oxidation of nitro to ketone with ozone.37... 

Diastereoselective hydride addition is quite versatile, and it provides facile synthetic access to ( —)-pinidine (661), an alkaloid isolated from several species of Pinus, as well as its unnatural isomer (+ )-pinidine (660b). The unstable aldehyde 655, prepared in four steps from 624 [202], undergoes Grignard addition with 4-pentenylmagnesium bromide followed by Swem oxidation to afford ketone 656 in 90% yield for the two steps. Stereoselective hydride addition with L-Selectride provides the -yn-alcohol 657 (91 9), while zinc borohydride reduction provides almost exclusively the anri-alcohol 658 (>99 1) (Scheme 144). 

C-Glycosidation of enol silane 279 to lactol acetate 278, prepared from 277 in two steps, furiushed ynone 280 as a single isomer. Reduction of the ketone with L-selectride furnished alcohol 270 with poor selectivity, but the minor isomer can be converted into the desired isomer via the Mitsunobu protocol Dihydroxylation of the terminal alkene, reduction of alkyne, and oxidative cleavage of the resulting triol gave the intermediate hydroxy aldehyde, which was spontaneously transformed into macrolactol 281 as a single diastereomer. 

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