Ring-opening recyclization

Pyrylium perchlorate, 4-chloromethyl-2,6-dimethyl-ring opening recyclization and, 3, 660... 

Other examples of nucleophilic attack on a furoxan ring leading to ring opening/recyclization are the formation of 1,2,3-triazole 1-oxides 198 from 4-alkylamino-3-nitrofuroxans 197 and alkylamines (Scheme 129). 3-Amino-4-nitrofurazan was observed as by-product (95MC194, 96CHE580, 96KGS675). 

Other examples of nucleophilic attack leading to ring opening/recyclization are the formation of triazolidinedione (70) from oxadiazolinone (62e) and methylhydrazine, presumably via intermediate (62f) <89JHC23l>, and the conversion of phenylhydrazones (62g) into triazinones (71) in the presence of base <92JHC959>. 

Type cycloadduct 24 (Scheme 1) was obtained as the major product on cycloaddition of n-chloranil with l-ethoxycarbonyl-l/7-azepine <1982H(19)1197>. Treatment of 4-ethoxycarbonyl-5-chloro-l,2,3-thiadiazole with ethylenediamine under basic conditions occurs with heterocyclic ring opening/recyclization to form bis(triazole)-fused thiadiazepine 77 (R = COOEt) (Scheme 14). The benzo-fused analog was prepared by the similar reaction with o-phenylenediamine <1999CC2273>. 

This is a simple example of a ring-opening/recyclization sequence to give y-butenolides. 

In their search for the optimal base for the selective formation of 139, the authors unexpectedly found a different reaction product, namely 142 (nine examples, 38-75% yield). This product is formed when a nucleophilic base such as sodium ethoxide or potassium ferf-butoxide was used instead of NEts (under otherwise identical reaction conditions as for the formation of 139). The formation of 142 is explained by nucleophilic attack of ethoxide or ferf-butoxide to intermediate 138 followed by a ring-opening/recyclization. 

Chebanov and co-workers [130] described an efficient synthetic route for the synthesis of some novel derivatives of 5a-hydroxy-4,5,5a,6,7,8-hexahydropyra-zolo[4,3-c]quinolizin-9-ones 94 which is based on a multicomponent condensation of a 5-aminopyrazole with a cyclic 1,3-diketone and an aromatic aldehyde under microwave irradiation. The reaction runs via an unusual base-mediated ring-opening/recyclization of the cyclic 1,3-diketone moiety (Scheme 71)... 

Scheme 11 Approach to oxazolidinones by a ring-opening-recyclization-cleavage procedure.

The versatility of Meldrum s acid as a synthon continues to be demonstrated for example, it is alkylated under phase transfer catalysis to give 5,5-dialkyl derivatives209 and it reacts with monohydrazones of 1,2-diketones to give alkylidenes (204) which undergo ring opening-recyclization to pyridazin-3-ones [such as (205)], 210 The catalysis of the hydrolysis of benzylidene-Meldrum s acid has been studied.211 New light has been thrown on the conformation of 2,2,4,4-tetramethyl-1,3-dioxans by a study of their 13C n.m.r. spectra.212... 

A library of pyrazoloquinolizinones has been prepared via a multicomponent reaction between 5-aminopyrazoles, cyclic 1,3-diketones, and aromatic aldehydes. Under the strongly basic conditions used, the initial product formed undergoes a subsequent ring-opening/recyclization process to give the pyrazoloquinolizinones (Scheme 4.14). 

Depurination. ring opening, recyclization, e.g., 5-membered pyrimidine Standard analysis... 

PET Carboxylic acid PET/EGMA Ring opening Recycling 233... 

A regiospecific ring-opening recyclization of -lactams gives 3-spirocyclicquinolin-4(lW)-ones (Scheme 72). ... 

Scheme 11.67 Metal carbene nucleosides via ring opening/recyclization.

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