Stereochemical Aspects of the “tert-Amino Effect”: 1. Regioselectivity in the Synthesis of Pyrrolo[l,2-a]quinolines and Benzo[c]quinolizines

Walter H.N. Nijhuis, Willem Verboom, A. Abu El-Fadl, Sybolt Harkema, David N. Reinhoudt

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    Abstract

    Substituted 2-vinyl-N,N-dialkylanilines cyclize in refluxing 1-butanol to give substituted pyrrolo[1,2-α] quinolines and benzo[c]quinolizines. This reaction proceeds via a 1,5-hydrogen transfer and subsequent C-C bond formation. When in the 2-vinyl-N,N-dialkylanilines 4, R1= H and R2 = H (4a,d), CH3(4b,e), or C2H5(4f), the cyclization products 5a,b,d-f are formed selectively, with the substituent R2at the bridgehead carbon atom. This regioselectivity is lost when R2 = CH2OCH3(4c,g), and a mixture of the regioisomers 5c,g, 6c,g, and 7c,g is formed. Reaction of compounds 4h-n (R1= CH3) yields the pyrrolo[l,2-α]quinolines 5-7(h-j) and benzo[c]quinolizines 5-7(k-n) selectively, in which the substituent at the bridgehead carbon atom is at the same face of the molecule (cis) as the hydrogen atom at C-5 [5-7(h-j)] or at C-6 [5-7(k-n)]. The configuration of these compounds was determined by1H NOE difference spectroscopy and single-crystal X-ray analysis (6n). Heating of 4o-q (R1= 4-C6H4CH3) in refluxing 1-butanol gives mixtures of the cis [5-7(o-q)] and trans [8—10(o—q)] compounds. The mechanism of these cyclizations, which are further examples of the “tert-amino effect”, and the effect of variation in substituents are discussed.

    Original languageEnglish
    Pages (from-to)199-209
    Number of pages11
    JournalJournal of organic chemistry
    Volume54
    Issue number1
    DOIs
    Publication statusPublished - 1989

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