The stereoselective synthesis of 2-substituted 3- azabicyclo[3.2.0]heptanes by 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 described. 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 recorded (22%). The facial diastereoselectivity of the reaction is high if α-branched N,N-diallylamines are employed as starting materials which either bear a comparably bulky α-substituent (25) or the α-substituent of 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 photocycloaddition could be conducted in two ways. The more general method is based on the Cu-catalyzed reaction which was carried out with Cu(OTf)₂ in diethyl ether and is applicable to essentially all substrates under scrutiny. The N-cinnamyl substituted substrates 5, 11, 16 and 25 which possess a comparably low lying T₁ state reacted well in a triplet-sensitized process employing acetophenone as the sensitizer and acetone as the solvent.