Carbon-acetal tether

To carry out tethered [3-i-2]-cycloadditions Nakamura et al. developed the new annulating reagents 123 [109, 110]. The reagents carry, in one molecule, two cyclopropenone acetals that are connected with an n-carbon methylene tether. Upon thermolysis in the presence of Cjq, the reagent undergoes [3-i-2]-cycloaddition reaction twice in a regio- and stereoselective manner to give Q- and C2-symmetrical bisadducts with cis-l - and cis-3-addition patterns, respectively. The selectivity varies... 

In the first phase the vinyl ether 51 is prepared from the acetate 50 by methylene-transfer from the Tebbe-reagent. Next, the thioglycoside 51 and the aglycone 52 are connected by acid-catalyzed addition of the latter to the vinyl ether moiety to afford the acetal-tethered intermediate 53. In the final phase, NIS/TfOH-induced anomeric activation proceeds in concert with the attack of the tethered oxygen at the anomeric carbon to afford the P-linked disaccharide 54 (Scheme 16) [114]. 

When a twofold molar excess of an aldehyde, ketone, ketonitrile, or vinyl acetate (the latter provides formaldehyde oxide +CH2-0-0 ) is co-ozonolyzed with various cycloalkene derivatives, three main products are obtained (1) an ozonide 83 with an aldehydic group tethered via an -carbon chain (2) a bicyclic tetraoxepane compound 84 formed from the above dipolar chain and the added carbonyl derivative and (3) a diozonide 85 resulted from the formaldehyde oxide and the aldehydic compound 83. Structures and yields of these products are presented in Scheme 25 and Table 9. 

The incorporation of a phenyl ring as part of an extended carbon tether [(C) , n — 2] requires the preparation of the homologous [2-[(E)-phenyl-diazenyl]phenyl]acetic acid 89 or its methyl ester 90 (Scheme 22), which in a subsequent step has to be transformed into l-diazo-3-[2-[(E)-phenyldia-zenyl]phenyl]acetone. 

In contrast, vinylsilane acetals (43a)-(43c), with a two-carbon tether between the vinylsilane and the MEM group, cyclize in the presence of dn tetrachloride to form tetrasubsdtuted alkylidenetetrahydro-furans (44a>K ). However, little or no stereochemical preference for either ( )- or (Z)-alkene stereoisomers was observed in these reactions (Scheme 20 and Table 4)." ... 

One of the reasons for its widespread use is based on the fact that CoA - and the CoA-derived phospho-pantetheine tether found on various carrier proteins - acts as the major carrier of metabolites containing carboxylic acid groups short and long carbon chain acids, Krebs cycle metabolites, and amino acids are the most common compounds carried. However, at any given moment most CoA molecules are involved in the transfer of acetyl groups among a variety of both small and macromolecules, and consequently it is from this role that the cofactor derives the name that Lipmann originally bestowed on it Coenzyme A, to indicate its involvement in activation of acetate. ... 

Nitrites. Allylic cyanides are formed from the acetates or carbonates in (PhsPjaPd-catalyzed transformations. Intramolecular cyanosilylation of alkynes provides a way of controlled functionalization of such compounds using a tether tac- a-Cyano selenides are products from SnCU-catalyzed reaction of diselenoacetals with MeiSiCN. Seleno-ortho esters also exchange one seleno group for the cyanide. ... 

Although the (bromomethyl)silyl ether connection has been more extensively utilized in radical cyclizations, Stork also introduced, at a similar time, the use of a mixed acetal function [51a, 73b, 80], This tether differs in that a two-carbon unit is introduced on the proximal carbon atom of the olefin, whereas the silyl tether allows the incorporation of only one carbon atom. The chemistry of this tether is dominated by 5-exo-trig cyclizations onto allylic double bonds which proceed with the usual degree of high stereoselectivity. 

The Pd-catalyzed cycUzation of alkenyl- and alkynyl-tethered allylic acetates gives a-alkyl- and alkenylpalladium intennediates, respectively, which can undergo further intramolecular alkene insertion, carbonylation, and transmetallation, respectively, to form additional carbon-carbon bonds (Scheme 8), as exemplified for such cascade reactions with insertion of a tethered alkene in Scheme 12.E3],[24H27]... 

Takahashi reported a novel Pd-catalyzed cyclization of allylic acetate and allenic moietyJ" The reaction is discriminated by the number of tethered carbon chains, as... 

Recently, palladium(II)-catalyzed arylation/carboxylation reactions were reported for the synthesis of carbazoles starting from 2-alkenyl indoles. These reactions employ the electron-rich C-3 carbon atom of the indole nucleus as carbon nucleophile in cyclization/carboalkoxylation reactions. The conditions that proved effective for 2-(2,3 or 5-alkenyl)-3-unsubstituted indoles are similar to those reported for the cyclization ofalkenes and alkynes bearing tethered heteronucleophiles. In particular, the best results has been obtained using PdCl2(CH3CN)2, CuCl2 in methanol under CO (latm). Under these conditions methyl (9-methyl-2,3,4,9-tetrahydro-4-carbazolyl)acetate 86 was isolated in 94% yield (Scheme 13.53) (100,101]. 

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