Acyloins cyclic

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

This modification has become the standard procedure for the acyloin ester condensation. By doing so, the formation of products from the otherwise competitive Dieckmann condensation (Claisen ester condensation) can be avoided. A product formed by ring closure through a Dieckmann condensation consists of a ring that is smaller by one carbon atom than the corresponding cyclic acyloin. 

N-Cyanoguanidine, 35, 69 Cyanohydrin formation, 33, 7 3-Cyano-6-isobutyl-2(l)-pyridone, 32, 34 3-Cyano-6-methyl-2(1)-pyridone, 32, 32 l-Cyano-3-a-naphthylurea, 36, 11 1-Cyano-3-phenylurea, 36, 8 Cyclic acyloins, 36, 82 Cyclization, (3-aminoethylsulfuric acid to ethylenimine, 30, 38 1,2-benzo-3,4-dihydrocarbazole from phenylhydrazine and a-tetralone,... 

The acyloin condensation was used in an ingenious manner to prepare the first reported catenane (see p. 91).727 The catenane (39) was prepared by a statistical synthesis (p. 91) in the following manner An acyloin condensation was performed on the diethyl ester of the C34 dicarboxylic acid (tetratriacontandioic acid) to give the cyclic acyloin 37. This was reduced by a Clemmensen reduction with DCI in D20 instead of HC1 in H20, thus producing a C34 cycloalkane containing deuterium (38) 728... 

The formation of a-naphthols of type 189 from 1-alkyl-substituted salts 30, on heating with dimethylamine hydrochloride in ethanol, occurs by another mechanism and will be explained in Section III,C,4,b,i. The interaction of l-aryl-3-carboxy-substituted salts 62 with secondary amines in benzene is initiated probably as in the reaction of these salts with primary amines, and the attack by the secondary amine on position 3 is the primary step of this reaction. However, since protonation of the intermediate anion 190, a masked acyl anion, becomes difficult, an interaction of this anion with the carbonyl group of the benzophenone fragment occurs (86KGS125). The enamines 191 thus formed are usually hydrolyzed on purification, yielding five-membered cyclic acyloins 192. 

Suzuki et al., we presented our observation that indeed various five- and six-membered cyclic acyloins could be obtained as their racemates employing commercially available thiazolium salts as precatalysts [34]. 

The acyloin condensation has been used with great success, in boiling xylene, to prepare cyclic acyloins from diesters.The yields are 50-60% for the preparation... 

D.J. Burnell et al. synthesized bicyclic diketones by Lewis acid-promoted geminal acylation involving cyclic acyloins tethered to an acetal. The required b/s-silyloxyalkenes were prepared by using the standard acyloin condensation conditions. ... 

The acyloin coupling reaction has also been used to prepare cyclic acyloins in which one of the ring atoms is an oxygen, sulfur or silicon atom. For seven- or eight-membered rings the use of TMS-Cl trapping may give improved yields. An example is shown in Scheme 20. ... 

Synthesis of m-cyano acids. Wasserman and Druckrey8-9 developed a novel method for the preparation of cu-cyano acids (IV), involving in the first step condensation of a cyclic acyloin (I, x — 4, 5, 6, and 10) with formamide in an acidic... 

Further, the fused-ring oxazoles (57) are readily prepared from the corresponding cyclic acyloins by condensation with formamide in acidic medium.94-126... 

When the cyclic acyloin, c-(CH2)iqCOCH(OH) was heated with formamide, in the presence of acid, a bicyclic oxazole, C13H21NO, was formed what is its structure This bicyclic oxazole was converted by exposure to O2, then heating, into the acyclic cyano-acid, H02C(CH2)iqCN draw a mechanism for the transformation. 

Acyloin condensation (2, 435-436 3, 311-312 4, 537). An example of the preparation of cyclic acyloins with trapping of the intermediate enediol with (CH0)SiCI has been published in Org. Syn. (equation I). ... 

When the cyclic acyloin, (CH9)jqCO.CH(OH) was heated with formamide,... 

Ethyl n-butyrate added dropwise to Na-sand, 2 moles of trimethylchloro-silane, and refluxing ether mixture of cis- and frans-4,5-bis(trimethylsiloxy)-4-octene (Y 92.3%) stirred and vigorously refluxed 1-2.5 hrs. on a water bath in a HGl-water-ether (or tetrahydrofuran) mixture n-butyroin (Y 96.2%). F. e. s. U. Schrapler and K. Riihlmann, B. 96, 2780 (1963) cyclic acyloins s. B. 97, 1383 (1964). 

In 2006, Enders reported an intramolecular cross benzoin reaction of bifunctional substrates 53 containing aldehyde and ketone groups to give various six-membered cyclic acyloins 54 in moderate to good yields. Up to 98% ee has been obtained upon catalysis by the tetracyclic triazolium salt 52 (Scheme 36.15) [21aj. Suzuki also developed a similar intramolecular asymmetric cross benzoin reaction [2lb]. 

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