Photochemical Synthesis and Some Reactions of 7‐Oxa‐and 7‐Thiatricyclo[3.2.1.0 3,6 ]octan‐2‐ones

Irradiation (λ = 350 nm) of newly synthesized 2‐acetyl‐ or 2‐methyl‐2‐(alk‐2‐enyl)furan‐3(2 H )‐ones 1 and 2‐acetyl‐ or 2‐methyl‐2‐(prop‐2‐enyl)thiophen‐3(2 H )‐ones 2 affords the corresponding 1‐acetyl‐ or 1‐methyl‐substituted 7‐oxa‐ and 7‐thiatricyclo[3.2.1.0 3,6 ]octan‐2‐ones 10 and 11 , respectively, via regioselective intramolecular [2 + 2] photocycloaddition in 65–95% yield ( Scheme 2 ). The 1‐acetyl‐substituted O‐derivatives 10b and 10c undergo ring opening on treatment with MeONa in MeOH at–78° to afford stereoselectively methyl 3‐ exo ‐acetyl‐2‐oxabicyclo[3.2.0]heptane‐7‐ endo ‐carboxylates 12b and 12c , respectively, while a 2:1 diastereoisomeric mixture of methyl 3‐acetyl‐2‐thiabicyclo[3.2.0]heptane‐7‐ endo ‐carboxylates 13 and 14 is obtained from the corresponding S‐derivative 11b . The outcome of the Huang ‐ Minlon reduction of the 1‐methyl‐substituted ketones 10a and 11a is again influenced by the heteroatom in the tricycle. While 1‐methyl‐7‐oxatricyclo[3.2.1.0 3,6 ]‐octane ( 15 ) is the only product from the corresponding oxatricyclooctanone 10a , a 1:2 mixture of 1‐methyl‐7‐thiatricyclo[3.2.1.0 3,6 ]‐octane ( 16 ) and 3‐methylbicyclo[3.1.1]hept‐2‐ene‐6‐ endo ‐thiol ( 17 ) is obtained from the analogous S‐compound 11a , both products stemming from a common carbanion precursor.