I cyclization

A variety of methods for the asymmetric syntheses of aziridine-2-carboxylates have been developed. They can be generally classified into eight categories based on the key ring-forming transformation and starting materials employed (i) cyclization of hydroxy amino esters, (ii) cyclization of hydroxy azido esters, (iii) cyclization of a-halo- and ot-sulfonyloxy-(3-amino esters, (iv) aziridination of ot, 3-unsaturated esters, (v) aziridination of imines, (vi) aziridination of aldehydes, (vii) 2-carboxylation of aziridines, and (viii) resolution of racemic aziridine-2-carboxylates. 

Monothiodiacylhydrazines 127, derived from the acylation of thiosemicarbazides or as intermediates in the reactions of (1) thiohydrazides with carboxylic acids and their derivatives (see Section 5.10.9.2.2(i)) or (2) hydrazides with thiocarbonyl compounds (see Section 5.10.9.2.3(i)), cyclize in the presence of an acid catalyst to give 1,3,4-thiadiazoles 128 (Equation 39, Table 4). 

Type I cyclizations are generally restricted to the formation of five-membered rings, whereas formation of six-membered rings occurred more readily than that of five- or seven-membered rings in type II cyclizations. Examples of intramolecular zinc-ene reactions of both types have been reported. 

Type zinc-ene reactions seem to be more difficult to achieve compared to type I cyclizations even though successful examples of six- or seven-membered ring formations have been reported. Nevertheless, type I reactions constitute a particularly attractive strategy for the formation of five-membered rings that proceed smoothly compared to intermolecular additions to monosubstituted alkenes. However, allylic organozincs are able to add to substituted alkenylmetals under extremely mild conditions. 

Tetrahydrofuranes.1 An example of a new route to these heterocyclcs is formulated in equation (I). Cyclization with acid alone is effected in only about 30%... 

CATEGORY lldf AND CATEGORY llf/i CYCLIZATIONS - DIELS-ALDER REACTIONS OF VINYLPYRROLES... 

Enantiomerically pure pipecolic acid (6) is accessible essentially by two well-established synthetic routes (i) cyclization of l- or D-lysine by reaction with disodium nitrosyl-pentacyanoferrate(II) with preservation of configuration at C2 215 216 (ii) ring closure of A ,Ae-bis(A-nitroso-A-tosyl) derivatives of l- or D-lysine, again with retention of chirality at C2. 217 Stereoselective synthesis of pipecolic acid derivatives, substituted in position 4, is achieved using the aza-Diels-Alder reaction of imines with dienes 218-220 or via an ene-iminium cyclization. 221 222 ... 

Kursanov, D. N., Fames, Z. N., Zaretskaya, I. I., Nazarov, I. N. Reaction mechanism of the cyclization by means of deuterium. I. Cyclization of isopropenyl allyl ketone. Bull. Acad.Scl. USSR, Chem. Scl. (English Translation) 1953, 103-107. 

AryltetralonesJ Cyclopropyl ketones such as i cyclize to 1-aryltetralones 2 and 3 in the presence of Lewis acid catalysts such as SnCl4, CF3COOH, and BF3 etherate. 

Table I - Cyclization ratios in free radical cyclopolymerizations.

I.5.5.3.I.I.I. Cyclization of Open-Chain Compounds to Five-Membered Rings... 

A number of rearrangements are observed in the coumarin system, e.g. the ring contraction of 3,4-dibromo-3,4-dihydrocoumarin 1 with alkali to give benzo[6]fiiran (coumarone, see p 64). The rearrangement of 4-chloromethylcoumarin 6 into coumarone-3-acetic acid 7 by aqueous alkali is mechanistically related to this reaction. This occurs via an S i cyclization of the phenolate 8 into the carboxylate 9 which then isomerizes ... 

Methylenebutyrolaetones. y, 8-Acetylenic carboxylic acids (I) cyclize to y-methylenebutyrolactones (2) in the presence of HgO when heated neat or in... 

Butenoate-phosphoranes 275 were prepared by the same route. The less stabilized ylides 275fj i were isolated as phosphonium salts after quenching with trifluoroacetic acid, but conversion to TFA salts was carried out also on 275a d in order to protect the phosphorane double bond during ozonolysis to 272. The free bases 272a d,f,i cyclized to the corresponding penems 276 upon mild heating in benzene. 

The Sonogashira coupling was applied to the synthesis of an indole-containing KDR kinase inhibitor by tandem Sonogashira coupling/5-ew[Pg.99]

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