Diborane amine derivatives

Reduction of a 20-N-benzyliminopregnane with diborane gives the 205- and 20R -benzylamines in 55 45 ratio. The corresponding imine (231) derived from the chiral (S)-(-)-a-phenylethylamine was reduced asymmetrically to give only the 20S-amino-derivative, which afforded the pure 20S-amine (232) after debenzyla-tion.201... 

Hydroboration of 11 -methylene-3,6-dimethylbenzomorphans (160) with diborane followed by H202 oxidation affords, stereospecifically, the corresponding 11/3-hydroxymethyl derivatives/104 105 If, however, the amine-borane complex (169) is heated to 140-150° prior to peroxide treatment, the corresponding 1 la-hydroxymethyl results in addition to the 11/3-epimer. Kugita and Takeda(106) propose stereocontrolled attack from the nitrogen side or /3-face (i.e., 171) by borane for the free base, but a-face attack by borane in the case of the amine-borane complex. 

Enamines may be reduced by sodium borohydride to give saturated amines, but only if a protonating species is available to convert the enamine initially into the iminium cation (lo). Steroidal g-amino-g,5-dienes are unreactive to sodium boro-hydride alone, but addition of acetic acid leads to rapid reduction via the iminium ion (10) to give gj -amino-A -steroids [224], The possibility that diborane was the reactive species in the NaBHd/HOAc system was excluded by the virtual non-reactivity of the enamine to externally generated diborane. Reduction of the iminium ion derivative (3) of a 6-formyl enol ether has been exploited in a variant of the Vilsmeier synthesis of 6-methyl steroids [22 ], the reduction product was the 6-aminomethyl enol ether (ii) which suffered hydrogenolysis with Raney nickel to give the 6-methylated enol ether (12). 

Curiously, 1,2-dinitrobenzene (25) couples with the loss of one nitro group however, the product is a diphenylhydroxylamine derivative (26) rather than a diphenylamine (equation 16). Nitroso compounds can also be reduced to amines in good yields using diborane (equation 17). ... 

Since hydroxylamines, as well as their precursor oxime derivatives, are reduced by diborane to amines, the reaction has been extended by reducing the initially formed hydroxylamine intermediates (57) to amines (58). Thus excess borane reduces nitroalkenes (56) to amines (58) in the presence of a catalytic amount of sodium borohydride (equation 32). 

A variety of reducing agents have been used for the conversion of imines to amines.The examples in equations (50) and (51) are typical. The relatively slow destruction of sodium cyanoboro-hydride in slightly acid aqueous solutions allows the process of imine formation to be carried out simultaneously with reduction. Apparently, the protonated imine is the species that is reduced and this process occurs significantly faster than reduction of the original carbonyl compound by this reagent. A recent report by Nose indicated that diborane in methanol will reduce only imines and no other functionality. Apparently, the diborane reacts rapidly with methanol to form a derived reagent. 

Reduction of chiral amines. Reduction of the 20-iminopregnane derivative (1) with either diborane or ( )-di-3-pinanylborane gives, after debenzylation, about equal amounts of the 20a- and 20/3-amine (2a and 2b). However, reduction... 

Amidation of 203 followed by partial hydrolysis affords D-jS-malamic acid (899). Reduction of 899 with diborane furnishes (R)-4-amino-l,2-butanediol (900). Protection of the amine with a Cbz group and conversion of the primary alcohol to a bromide leads to the critical intermediate 901. Alkylation of lysine derivative 902 with 901 and removal of the protecting groups under hydrogenolytic conditions gives hypusine (903). 

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