SYNTHESES OF 3-PHENYL SUBSTITUTED INDOLIZIDIN-2-ONES AND A PYRROLIZIDIN-2-ONE ON THE ROUTE TO CONSTRAINED POTENTIAL NK1 RECEPTOR ANTAGONIST

5-Benzoyl substituted isoxazolidines 3a,b and 4a,b obtained by 1,3-dip olar cycloadditions of pyrroline N-oxide and tetrahydropyridine N-oxid e to phenyl vinyl ketone, undergo a rearrangement to 3-phenyl-2,5,6,7- tetrohydro-3H-pyrrolizin-2-one and 3-phenyl-2,3,5,6,7,8-hexahydroindol izin-2-one, catalysed by Al2O3, followed by a Michael addition to a se cond molecule of phenyl vinyl ketone. Isoxazolidines 3a,b, in presence of an excess of phenyl vinyl ketone, undergoes another rearrangement to a 6,7-dibenzoylindolizidin-7-ol (21) by a non-reductive isoxazolidi ne ring cleavage. Isoxazolidines 4a,b do not undergo this second rearr angement, but give the expected 3-phenyl-indolizidin-2-one (2), as a s ingle diasteroisomer, by reductive N-O bond cleavage with Mo(CO)(6). T he reaction mechanism for the new arrangements are discussed.