Silver compounds rearrangement

The skeleta of other strained systems such as cubanes and related compounds can also be reorganized in the presence of silver salts. Rearrangements of cubanes were described in the early 1970s. Eaton et al.35 showed that mono- or disubstituted cubanes gave a mixture of polycyclic regioisomers (Scheme 3.21). No yields were given. 

Hiyama et al.69 showed that monoacetylated 1,4-butynediols nicely afforded acetoxy allenols on silver-catalyzed rearrangement. These compounds were ideal substrates for silver-catalyzed cyclization, so the overall sequence could directly be performed in one pot, leading to substituted 2,5-dihydrofurans in high overall yields (Table 3.5). In this process, 5-10 mol% of silver perchlorate or tetrafluoroborate was used in refluxing benzene. 

This ester is one carbon atom short of the full side chain of grandisol, so an Arndt-Eistert reaction was used to lengthen the chain by one atom. First, the ester was converted into the diazoketone with diazomethane and, then, the Wolff rearrangement was initiated by formation of the carbene with a Amdt-Eistert chain extension of ester silver compound at the Ag(II) oxidation state. 

Sigmatropic rearrangement 677, 697 Sihca gel membranes 1082 Silphinene, electrosynthesis of 1183, 1187 Silver compounds, as oxidants 1307-1311 Silybin, synthesis of 1308, 1310 Silydianin 1180 Silylation, of phenols 934, 935 Silylcyanation, asymmetric 694, 695, 702 Simmons-Smith cyclopropanation, asymmetric 694... 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

The use of copper as a catalyst in carbenoid transfer has its roots in the Amdt-Eistert reaction, Eq. 1 (3). Although the original 1935 paper describes the Wolff rearrangement of a-diazo ketones to homologous carboxylic acids using silver, the authors mention that copper may be substituted in this reaction. In 1952, Yates (4) demonstrated that copper bronze induces insertion of diazo compounds into the X-H bond of alcohols, amines, and phenols without rearrangement, Eq. 2. Yates proposal of a distinct metal carbenoid intermediate formed the basis of the currently accepted mechanistic construct for the cyclopropanation reaction using diazo compounds. 

A density functional study has been made of the competition between Wolff rearrangement and [1,2]-H shift in /S-oxy-a-diazocarbonyl compounds. Silver-catalysed decomposition of a-diazoketones (88 n = 0), derived from A-tosyl a-amino acids in methanol, gave rise to mixtures of products of Wolff rearrangement (89) and direct insertion of the carbene into the NH bond (90). The -amino acid derived species (88 n = 1) gave rise to products of Wolff rearrangement. 

A nice example of a degenerate rearrangement has been reported for the complex pentacarbonyltungsten(0)-thioaldehyde-l,2-dithiol 177 (R = H) or the thioketone-1,2-dithiol complex 177 (R = CH3). These compounds (obtained by the reaction of silver nitrate with tetraethylammonium pentacar-bonyliodotungstate(O) in the presence of l,6,6 z -trithiapentalenes) appeared to exist as an equilibrium mixture of the two isomers 177/178 (Scheme IV.70) [83JCS(CC)289]. The symmetrically substituted 1,2-dithiol (R = CH3, R = H) displays fluxional behavior as appears by NMR spectroscopy... 

As a consequence of the cyclobutyl to homoallyl rearrangement, reactions of 1-chloro-, 1-bro-mo-l,2-dimethylcyclobutane, 1-chloro- or l-bromo-l,3-dimethylcyclobutane with silver(I) te-trafluoroborate in diethyl ether gave in each case the same open-chain compound 2-fluoro-4-mcthylpent-4-ene (14) as the major product. Thus, l-bromo-l,3-dimethylcyclobutane (13) was treated with silver(l) tetrafluoroborate in diethyl ether at 0 >C to give 2-fluoro-4-methylpent-4-ene (14).22... 

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