Condensation of Oxazoles with Dienophiles — a New Method for the Synthesis of Pyridine Bases

Abstract

The condensation of oxazoles with dienophiles is of the diene synthesis type, the final reaction products being substituted pyridine bases. The review deals with the literature data concerning the preparative applications of the condensation and the elucidation of the theoretical features of this process. In conclusion of the review, the application of heterodiene synthesis with participation of oxazoles in the preparation of vitamin B 6 and its analogues is discussed. The bibliography comprises 38 references. CONTENTS

Organocatalytic Enantioselective One‐Pot Four‐Component Ugi‐Type Multicomponent Reaction for the Synthesis of Epoxy‐tetrahydropyrrolo[3,4‐b]pyridin‐5‐ones

“…Deprotonation of the α‐proton by phosphate followed by the attack of the resulting enolate oxygen atom at the nitrilium carbon atom would afford oxazole 4 with the concurrent regeneration of the catalyst 7 g . Following addition of acyl chloride 5 and triethylamine, acylation of the secondary amine would take place to produce 11 , which would undergo intramolecular Kondrat′eva type Diels–Alder reaction to afford epoxy‐tetrahydropyrrolo[3,4‐ b ]pyridin‐5‐ones 6 15. From the stereochemical outcome that we observed, it was assumed that the cycloaddition went through a concerted amide‐ exo /R 3 ‐ endo mode and that the bulky R 1 group adopted a pseudo‐equatorial position to avoid the steric interference with both oxygen and the N ‐R 2 group.…”

Tilcotil® Studies. Part 2. [4 + 2] Additions with Isothiazol‐3(2H)‐one 1,1‐Dioxide

“…With respect to regiospecificity, it would seem justified to expect the reaction to follow the pattern observed previously with the dienes of Scheme 3. There is also precedence [35] for the fact that with unsymmetrical dienophiles, the most electronegative center ends up in position 3 of the adduct (and thus in position 4 of the rearranged pyridine derivative). This behavior can be rationalized in terms of frontier MO theory [35].…”

One‐pot synthesis of pyridine derivatives via diels‐alder reactions of 2,4‐dimethyl‐5‐methoxyoxazole

“…Thus thermal reaction of oxazoles with acetylenes leads to furans, while addition to olefins produces substituted pyridines [7]. In a few instances, the DielsAlder adduct was isolated [8,9] but in most cases the primary adduct was not isolable and instead underwent rearrangement [10,11]. The methodology was applied to the synthesis of natural products (e.g.…”