Silver compounds Silyl enol ethers

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Friedel-Crafts alkylation has been used in an important synthesis of aryl C-glycosides, which are potent anti-tumor agents, from glycosyl fluorides (equation 99)65 661. The reaction takes place rapidly in dichloromethane, at room temperature using a novel zirconium complex and silver perchlorate combination catalyst. A similar alkylation has been performed by replacing the aromatic compound with either a silyl enol ether or an allylic compound using silver triflate as the catalyst662,663. 

Treatment of 1,3-dicarbonyl compounds with DBP in a methoxide/methanol system affords 2-alkyl-4-[(phenylsulfonyl)methyl]furans, where reaction proceeds by Initial addition-elimination on the vinyl sulfone moiety. In contrast, silyl enol ethers in the presence of silver tetrafluoroborate resulted in products derived from Sn2 displacement at the allylic site.11 Anions derived from 1,3-dicarbonyls substituted at the C-2 position are found to induce a complete reversal in the mode of ring closure.12 The major products obtained are 3-[(phenylsulfonyl)methyl]-substituted cyclopentenones. The internal displacement reaction leading to the furan ring apparently encounters an unfavorable Ai -interaction in the transition state when a substituent group is present at the 2-position ol the dicarbonyl compound. This steric Interaction is not present in the transition state leading to the cyclopentenone ring. 

Padwa and coworkers applied the silver(I)-promoted alkylation of silyl enol ethers to synthesis of substituted furans [19]. For example, treatment of various trimethylsilyl enol ethers of cycloalkanones 44 and (F)-2,3-diiodo-l-(phenylsulfonyl)-l-propene (45) with 2 equiv. AgBF4 gives alkylated products 46 resulting from Sn2 displacement of the terminal iodide. These compounds 46 further cyclize with triethylamine to produce the 2-phenylsulfonylmethyl substituted furans 47 (Sch. 10) [19a]. 

This reaction constitutes a method for the transformation of saturated cyclic or acyclic carbonyl compounds in three steps (silyl enol ether formation, halocarbene addition, rearrangement) to a,) -unsaturated carbonyl compounds with one-carbon ring enlargement or chain elongation, respectively. The rearrangement can be induced under acidic, basic, or thermolytic conditions, or with silver(I) salts. 

Ketone silyl enol ethers react with derivatives of diacyl azo compounds at room temperature245 or on heating242,243 (see also Eq. 82) as well as enantio-selectively under the influence of silver trifluoromethanesulfonate and BINAP (Eq. 93)244 or copper bis(oxazoline) complexes (Eq. 94). The latter is proposed to proceed via a formal hetero Diels-Alder adduct.252 Ketones themselves react with azodicarboxylic esters either thermally246,389,390 or in the presence of potassium carbonate390 but yields are low. Higher yields can be achieved with LDA,391 394 (see also Eq. 88), LiHMDS,395,396 or KOBu-r325 as the bases. Aryl diazonium... 

Carbon-carbon bonds can be similarly formed at the anomeric centerof carbohydrates. Oxonium ions, formed by the treatment of l-(2 -thiopyridyl)glycosides v/ith Silver(I) Trifluoromethanesul-fonate, can be trapped with silyl enol ethers, silyl ketene acetals, and reactive aromatic compounds, where the stereoselectivity of the addition is determined by solvent and nucleophile choice. The intramolecular version of this process has also been examined (eq 5). Similarly, bicyclic piperazinediones are available by the intramolecular trapping of iminium ions, generated from the appropriate thiopyridyl derivatives with PhHgC104 (eq 6). ... 

Diketones often are protected as enol ethers or enamines and these selectively functionalized compounds may be subjected to complementary transformations (Scheme 94). Also silylenes can be prepared from diketones and -hydroxycarbonyl compounds by reaction with dimethyldicyanosilane. Naturally, these blocking groups are relatively sensitive to hydrolysis. On the other hand, partial solvolysis can open a route to monoprotected derivatives (e.g. 101), usually blocked at the sterically less demanding carbonyl function as 0-silyl cyanohydrins (see Scheme 95). Deprotection is finally achieved with silver fluoride in THF. 

A 2 1 mixture of silver carboxy late and iodine is well known to be a good oxidant in the transformation of enol silyl ethers into the corresponding a-acyloxy carbonyl compounds (139). Five- and six-membered ring enol silyl ethers serve as the best substrates, with larger ring systems forming a-iodo carbonyl compounds as byproducts. 

Various methods for the a-hydroxylation of carbonyl compounds have appeared. lodosobenzene shows a high degree of selectivity in the preparation of acyloins, and benzeneseleninic anhydride appears to be an excellent reagent for the hydroxylation at the tertiary centre in a-alkylcycloalkanones. Enol silyl ethers give acyloins with osmium tetroxide and V-methylmorpholine V-oxide,and a-acyloxycarbonyl compounds with silver acetate and iodine. a-Acyloxyketones may also be prepared conveniently by treatment of oximes with acid chlorides [equation (42)]. ... 

---