1,2-Dihydropentalenes from Fulvenes by [6 + 2] Cycloadditions with 1-Isopropenylpyrrolidine

Abstract

In situ generated acetone pyrrolidine enamine undergoes [6 + 2] cycloadditions with fulvenes to give 1,2-dihydropentalenes. This ring annulation method works particularly well with 6-monosubstituted fulvenes and is subject to steric hindrance at C-6 of the fulvene. On the basis of mechanistic studies, optimal conditions have been developed for a one-pot synthesis of 1,2-dihydropentalenes using catalytic amounts of pyrrolidine.

Diverse Modes of Reactivity of 6‐(Chloromethyl)‐6‐methylfulvene

“…When the above reaction was repeated under the same condition, but at 60 °C, in addition to small amounts of 22 , the 1,2‐dihydropentalene 23 was formed as the major product (33 % after chromatography). This result was expected based on our recent report on the formal [6+2] dipolar cycloadditions of fulvenes with enamines leading to 1,2‐dihydropentalenes . Compound 1 obviously constitutes a special case in that it not only contains an electrophilic fulvene C‐6, but also the CH 2 Cl that is not only electrophilic, but also a C‐H acid (Scheme ).…”

“…Substrate 1 was also of interest in terms of its reactivity toward the enamine derived from acetone. In a recent communication we reported on the formal [6+2] dipolar cycloadditions of fulvenes with acetone enamine leading to 1,2‐dihydropentalenes . The question whether 1 would react with a carbon nucleophile such as N ‐isopropenylpyrrolidine (formed in situ from pyrrolidine and acetone) at C‐6 leading to a 1,2‐dihydropentalene, or at the chloromethyl group in an S N 2 reaction was readily tested when the title compound was treated with 20 % pyrrolidine in acetone.…”

Pyrrolidine catalyzed reactions of cyclopentadiene with α,β-unsaturated carbonyl compounds: 1,2- versus 1,4-additions

“…Finally, the Michael adducts from three benzalacetones, 14a,b , 16a,b and 18a,b were converted to known 1,2-dihydropentalenes (obtained from a formal dipolar 6+2 cycloaddition of fulvenes onto N-isopropenylpyrrolidine) 21 by intramolecular condensation of the cyclopentadiene with the carbonyl group was readily accomplished with 10% pyrrolidine in MeOH.…”

Temperature-dependent, competitive 1,3-acyl shift versus decarbonylation of a cyclopropanone intermediate

“…1,2-Dihydropentalene 5 used in this study was prepared by a method we recently developed. 7 Exact masses of all new products by high resolution mass spectra (HRMS) were determined at the University of Notre Dame Mass Spectrometry Facility (http://www.nd.edu/~massspec/) and San Francisco State University Mass Spectrometry Facility.…”

“…Toward achieving our objectives we studied the photooxidation chemistry of 1,2-dihydropentalenes (1,2-DHPs), the only fulvenic DHP isomers, for whose synthesis we recently developed an efficient one-step method starting from fulvenes. 7,8 .…”