The sodium borohydrideltrif luoroacetic acid reduction of the title alkaloid (1) is highly stereoselective, giving the all-cis product 2 in excellent yield.Recent years have seen great improvements in the methodology for reducing indoles to dihydraindoles (indolines).These newer methods, generally under acidic conditions, with borane,' amine-borane~,~ boron h y d r i d e ~, ~ and trialkylsilanes, 5 supersede the older methods (i.e., metals in mineral acid, catalytic hydrogenation)' far generality, convenience, and efficiency.Furthermore, the use' of trifluoroacetic acid in several of these methods 293dv4b,4c,4e,5b allows one to reduce 1,2,3,4-tetrahydro-Mand y-)-carbolines and other indoles containing a second, basic nitrogen.These particular indoles can be difficult to reduce under weaker acid conditions because protonation of the basic nitrogen suppresses protanation of the indole double band, which is presumed necessary for successful reduction.For example, sodium borohydride (NaBH4) in trifluoroacetic acid (CF3C02H) has been used to reduce several indole[2,3-alquinolizidines and other indole alkaloids (i.e., yohimbine,7 r e s e r p i ~i n e , ~~ ibogaine,7b t e t r a ~h y l l i n e , ~~ r a u b a ~i n e ~~) .Although these reductions were reported to be stereoselective, no attempt was made to ascertain the stereochemistry of the major product .7bTo answer this question we 8 have studied the reaction of the readily available alkaloid 1,2,3,4,6,7,12,12boctahydroindolo[2,3-alquinolizine (119 with NaBH4ICF3CO2H. 10,ll Treating a solution of 1 in neat CF3C02H at -5' to 0' with NaBH4 (pellets) gave 1,2,3,4,6,7,7a,12,12a,12b-decahydroindolo~2,3-alquinolizine (2) as one major 1 stereoisomer, mp 75-75.5", in 90% purified yield.12913The H-nmr spectrum of 2 reveals one distinguishing feature: a one-proton doublet of doublets at 3.55 ppm 1.This paper i s P a r t X i n t h e s e r i e s "Reactions of Sodium Borohydride i n Acidic
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Gribble et al. (1981) studied this question.