Heterocyclization Reactions of Conjugated Heterocumulenes. Synthesis of Pyridine Derivatives by a Tandem Aza Wittig/Electrocyclization Strategy

Abstract

The aza Wittig reaction of iminophosphorane 3 with isocyanates leads to conjugated carbodiimides 4, which undergo electrocyclic ring closure to give ethyl 2-pyridinecarboxylates 5. Iminophosphorane 9 reacts with nitromethane and acetone in a 1 : 1 molar ratio to form 10 and 11, respectively. Reaction of 9 with nitromethane in 1 : 3 molar ratio yields the iminophosphorane 12. The structure of 12 has been established by X-ray crystallography. Iminophosphoranes 10 and 11 react with isocyanates or carbon dioxide to give the pyrazolo [3,4b] It has become increasingly apparent that a$-unsaturated heterocumulenes are highly useful as synthetic intermediates in preparative heterocyclic chemistry. Especially cycloaddition reactions of such unsaturated heterocumulenic systems as, e. g., ketenes, isocyanates, isothiocyanates, and ketene imines provide an attractive plex of thermally induced 671 electrocyclizations of conjugated heterocumulenes have been documented 'I; only the thermal cyclization isocyanates6', 0-carbarnoylvinyl isocyanates 'I, and P-substituted vi-with heterocumulenes, e. g. carbon dioxide, carbon disulfide, and isocyanates or isothiocyanates, is a very useful reaction in synthetic heterocyclic chemistry'! Consequently, improvements which increase the efficiency or enlarge its applicability are always desirable, ing a moiety able to react with the aza Wittig product is important in this respect., Continuing our interest in the iminophosphorane-mediated synthesis of heterocycles, we describe a simple and apparently general procedure for the synthesis of conjugated heterocumulenes and the first synthesis of pyridine derivatives by electrocyclic ring closure of conjugated carbodiimides, isothiocyanates, and isocyanates. and ethyl azidoacetate or from azidoformylazoles and activated methylene compounds. R2$R4entry to a variety of heterocycles? However, relatively few exarn-

An Efficient Synthesis of α‐Isothiocyanato‐α,β‐unsaturated Esters from Morita–Baylis–Hillman Adducts

“…The synthesis of 3 has been reported by sequential Knoevenagel condensation of aldehyde and methyl azidoacetate to form the azide derivative, a subsequent Staudinger reaction with PPh 3 to form iminophosphorane, and the aza ‐Wittig reaction with carbon disulfide (Eq. (3) in Scheme ) . However, the yields of α‐isothiocyanato‐α,β‐unsaturated esters were low to moderate in most cases (49–78%) 3a,3c,10a …”

“…(3) in Scheme ) . However, the yields of α‐isothiocyanato‐α,β‐unsaturated esters were low to moderate in most cases (49–78%) 3a,3c,10a …”

Synthesis of Pyrazolo[3,4‐b]pyridin‐6‐ones

“…The publication [51] is devoted to a convenient method of synthesizing derivatives of pyrazolo[3,4‐ b ]pyridine based on the reaction of imino phosphoranes ( 117 ) and ( 119 ). It is shown that iminophosphorane ( 116 ), synthesized from azidopyrazole ( 115 ) and triphenylphosphine [52], reacts with nitromethane in the presence of pyrrolidine with the formation of a nitrovinyl derivative ( 119 ; 55% yield) or it reacts with acetone under similar conditions with the formation of an iminophosporane ( 117 ; 50%).…”