Efficient and Practical Method for Synthesizing N-Heterocyclic Compounds Using Intramolecular Nucleophilic Acyl Substitution Reactions Mediated by Ti(O-i-Pr)₄/2i-PrMgX Reagent. Synthesis of Quinolones, Pyrroles, Indoles, and Optically Active N-Heterocycles Including Allopumiliotoxin Alkaloid 267A

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Abstract

Treatment of N-(2- or 3-alkynyl)amino esters with a low-valent titanium reagent diisopropoxy(η2-propene)titanium (1), generated in situ by the reaction of Ti(O-i-Pr)4 and 2i-PrMgCl, resulted in an intramolecular nucleophilic acyl substitution (INAS) reaction to afford α-alkylidene-pyrrolidinones or -piperidinones. Thus, treatment of N-propargyl-anthranilates 5, -indole-2-carboxylates 10, or -pyrrole-2-carboxylates 13 with 1 gave 4-quinolones 7, [1,2-a]indoles, or [1,2-a]pyrroles, respectively. Similarly, N-alkynylated α- or β-amino esters 14 or 15 with 1 afforded N-heterocycles 18 or 19. In the reaction of N-(2- or 3-alkenyl)amino esters with 1, the resulting INAS product underwent intramolecular carbonyl addition (ICA) reaction to afford the N-heterocyclic compounds having a cyclopropanol moiety in good to excellent yields. Thus, the treatment of N-alkenyl-anthranilate 4a, -indole-2-carboxylates 8 and 9, or -pyrrole-2-carboxylates 11 and 12 with 1 gave the corresponding quinoline derivative 6a, [1,2-a]indoles, or [1,2-a]pyrroles, respectively. The optically active N-heterocyclic compounds 20 and 21 were obtained from N-alkenylated α- or β-amino esters 16 or 17. A highly efficient total synthesis of allopumiliotoxin alkaloid 267A has also been accomplished. Thus, the N-propargyl-2[(1-hydroxy-1-methoxycarbonyl)ethyl]pyrrolidine 24 (from l-proline in six steps) reacted with 1 to afford the corresponding indolidinone 25 in 67% yield, which has previously been converted to allopumiliotoxin 267A.

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