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Hetero‐Diels–Alder Reaction with Thiazolyl Oxabutadienes—Model Studies Toward the Synthesis of Directly Linked C ‐Disaccharide
Author(s) -
Dondoni Alessandro,
Kniezo Ladislav,
Martinkova Miroslava,
Imrich Jan
Publication year - 1997
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19970030314
Subject(s) - chemistry , cycloaddition , double bond , hydroboration , alkene , stereochemistry , selectivity , moiety , medicinal chemistry , hydroxylation , pyranose , catalysis , organic chemistry , enzyme
A method for the construction of substituted pyranoses by means of the hetero‐Diels–Alder (HDA) reaction of ethyl vinyl ether with 1‐oxabuta‐1,3‐dienes bearing a thiazolyl ring at C‐2 is described. The cycloaddition with 1‐(thiazol‐2‐yl)‐2‐penten‐1‐one ( 2 ) occurred with good endo/exo selectivity to give cis ‐ and trans ‐3,4‐dihydro‐2 H ‐pyrans 3a and 3b in a ca. 9:1 ratio and 91% overall yield. The elaboration of 3a through the conversion of the thiazole ring into the formyl group and reduction of the latter to alcohol, followed by hydroxylation of the double bond through hydroboration–oxidation led to the ethyl 2,3‐dideoxypyranoside 8 . The asymmetric version of this synthetic sequence started from the HDA cycloaddition of the same alkene with the chiral oxabutadiene 10 bearing the D‐ galacto ‐pentopyranosid‐5‐yl moiety at C‐3. This reaction afforded a mixture of the four diastereomeric cycloadducts—3,4‐dihydro‐2 H ‐pyrans 11a,b and 12a,b —in 97% overall yield. The reaction was moderately endo and face selective. A high level of endo selectivity (96%) was obtained by the use of catalytic Eu(fod) 3 . The elaboration of the endo cycloadducts 11a and 12a by the same synthetic sequence as that developed for 3a (i.e. thiazolyl‐to‐formyl conversion and hydroxylation of the double bond) gave the uncommon C ‐disaccharides 15 and 16 featuring two directly linked pyranose rings.