Isoxazolidines by cycloadditions of N,α-diphenyl nitrone in the benzo[b]thiophene S-oxide and SS-dioxide series
1,3-Dipolar cycloadditions of N,α-diphenyl nitrone have been investigated in the 2- or 3-substituted (methyl, phenyl, chloro, bromo, piperidino, acetyl) benzo[b]thiophene S-oxide and SS-dioxide series. The S-oxide and SS-dioxide derivatives show the same ability to form adducts. The 2,3-dihydrobenzo[b]-thieno[2,3-d]isoxazolidines are generally formed only as a single regioisomer, except in the case of 2-methyl dipolarophiles where both regioisomers are formed. The regioselectivity is discussed in terms of frontier orbital interactions on the basis of CNDO/S calculations and photoelectronic spectral ionization potentials. The nitrone, which has an elbow shape, could lead to the formation of diastereoisomers but in fact only the trans-isoxazolidines are formed. There is less stereoselectivity in the S-oxide series since we obtained only one adduct with the 3-methyl dipolarophiles but both syn- and anti-epimers with the 3-phenyl derivatives. The bromo derivatives do not lead to any adduct. X-Ray structures are presented for the two adducts corresponding to the addition of the nitrone to benzo[b]thiophene SS-dioxide and to the 2-methyl derivative.