Synthesis of Pyrroles from 1-Dialkylamino-3-phosphoryl(or phosphanyl)allenes through 1,5-Cyclization of Conjugated Azomethine Ylide Intermediates

Abstract

1-Dialkylamino-1,3-diaryl-3-diphenylphosphanylallenes 3a-e are thermally converted into a-annulated 3,5-diarylpyrroles 6a-f and [a]-annulated benzo[c]azepines 7a,b,d. These transformations are likely to include conjugated azomethine ylide intermediates that can undergo either a 1,5- or a 1,7-electrocyclization. The periselectivity is markedly shifted toward 1,5-cyclization when the diphenylphosphanyl substituent is replaced by the diphenylphosphoryl group. Thus, 1-dialkylamino-3-(diphenylphosphoryl)allenes 4a-f yield pyrroles 6 exclusively and with improved yields, unless the 3-aryl substituent in the allene is too electron-rich (e.g., benzodioxol-5-yl, 4f --> 7f). The preparation and thermal transformation of aminoallenes 4 over three or four steps can be conducted as a one-pot procedure, thus providing a convenient synthesis of [a]-annulated 3,5-diarylpyrroles from enaminoketones.

Conjugated Azomethine Ylides

“…One example is presented in Scheme 13 (the PPh 2 substituent of 62 was oxidized to the phosphane oxide to allow the chromatographic separation). [30] It is known that under flash vacuum pyrolysis (FVP), methylene Meldrum's acid derivatives lose acetone and carbon dioxide to generate methylene ketene intermediates. Starting with N,N-disubstituted aminomethylene derivatives of Meldrum's acid (63), FVP allows the generation of the conjugated azomethine ylides 65.…”

Synthesis and reactivity of 3-(dialkylamino)allenyl phosphonium salts. PPh3-mediated synthesis of pyrroles from propyne iminium triflates

“…The reaction was also successful when PBu 3 was applied, furnishing salt 8 which was also prepared by in-situ hydrolysis of the allene formed from 1a and PBu 3 . In contrast to the hydrolysis of aminoallenes 2 and dicationic iminium salts 5, the alkynone route provided either the E or the Z isomer or mixtures of both (Table 2).…”

“…propyne iminium triflates react with neutral phosphorus nucleophiles such as Ph 2 P-SiMe 3 and Ph 2 P-OSiMe 3 in a conjugate addition to form ultimately (dialkylamino)allenyl phosphanes and phosphanoxides, respectively. 2 Under thermal conditions, these aminoallenes are readily hydrolysis has been mentioned.…”

“…In fact, salts 5 were occasionally found as by-products in the preparation of 2 from 1 when the latter were contaminated with traces of triflic acid. More conveniently than from allenes 2, salts 5 can be prepared in one step from propyne iminium salts 1 by simultaneous or successive addition of PPh 3 and triflic acid at low temperature. While salts 5a,b,g could be isolated in 72-94% yield, this was not the case for cyclopropyl-substituted salt 5d.…”

“…Recently, a direct Cu(I)-mediated conversion of alkynyl imines into pyrroles has been described; however, it is limited to substrates bearing a CH 2 group at the acetylenic bond. 16 The results reported in Table 4 are commented as follows: a) Although PPh 3 seems to furnish somewhat lower yields in comparison to Me 3 SiO-PPh 2 /LiCl as activator, 3 (e.g. 10a: 13 vs. 24%), it has the advantage of being the more convenient reagent.…”