The stereoselective synthesis of 2-substituted 3-azabicyclo[3.2.0]heptanesby intramolecular [2+2]-photocycloaddition reactions

The stereoselective synthesis of the 3-azabicyclo[3.2.0]heptanes 6, 12, 13, 17, 22, 23 and 24 by an intramolecular [2+2]-photocycloaddition is describ ed. The product yield was good to excellent in almost all cases studied (65 -87%) except for a single example (21 --> 24) in which a low yield was reco rded (22%). The facial diastereoselectivity of the reaction is high if alph a-branched N,N-diallylamines are employed as starting materials which eithe r bear a comparably bulky alpha-substituent (25) or the alpha-substituent o f which is connected to the nitrogen atom via an oxazolidinone ring (16, 19 , 20, 21). These diallylamines were readily prepared from methionine and it was shown that their preparation proceeded free of racemization. The photo cycloaddition could be conducted in two ways. The more general method is ba sed on the Cu-catalyzed reaction which was carried out with Cu(OTf)(2) in d iethyl ether and is applicable to essentially all substrates under scrutiny . The N-cinnamyl substituted substrates 5, 11, 16 and 25 which possess a co mparably low lying T-1 state reacted well in a triplet-sensitized process e mploying acetophenone as the sensitizer and acetone as the solvent.