A ring expansion via

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

Gree and co-workers have found that bicyclic epoxy alcohols when treated with diethylamino sulfur trifluoride (DAST) can undergo a ring expansion via C-C bond cleavage of the oxirane ring to provide fluorinated cyclic vinyl ethers (Table 17) <20020L451>. 

A single quinolone-fused oxepine 45 was reported in a study on the gold (I)-catalyzed formation of functionalized quinolones (130L4234).The starting 2-alkynylarylazide derivatives such as 44 undergo a 1,3-acetoxy shift, followed by cyclization of the azide and a ring-expansion via a 1,2-shift. 

Cyclic a-halomethyl or a-phenylselenenylmethyl (3-ketoesters undergo one-carbon ring expansion via transient cyclopropylalkoxy radicals.360... 

Although the merits of the synthetic method described in the preceding section are apparent, the limited applicability of this method prompted the search for alternative synthetic routes. During our work in this field we have found that a well known ring expansion via a carbene intermediate 45) does indeed proceed remarkably well with a variety of precursors. 

Figure 5.23 SAND can be used to modify amine-containing molecules, and then photo-initiate crosslinking to another amine-containing molecule via a ring-expansion process. The conjugates may be disrupted by reduction of the cross-bridge disulfide with DTT.

Figure 5.32 APDP can modify sulfhydryl-containing compounds through its pyridyl disulfide group to form disulfide bonds. Its phenyl azide end then can be photolyzed with UV light to couple with nucleophiles via a ring-expansion process. The disulfide group of the crosslink can be selectively cleaved using Dll.

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

During the synthesis of ligands for the vesicular acetylcholine transporter, a octahy-droquinoline nucleus 361 was prepared via a ring-expansion rearrangement followed by a hydride reduction (equation 140). The yield was moderate but only one lactam 360 was produced by the Beckmann rearrangement. 

The thiazolium salt (399) undergoes base-induced ring expansion via (400) and (401) to give (402) (80TL2429), a direct parallel of the conversion of analogous oxazolium salts into 1,5-benzoxazepines. 

The isoxazolidines derived from aldehydes (17a-b) were converted to eudesmane sesquiterpenes. Thus, quatemization of isoxazolidine (18) was followed by ring expansion via a Stevens rearrangement. 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

The observation of the silacyclohex-3-ene 83 was rationalized by a [2+2] cycloaddition of ethylene to give unusual 2-vinylcyclobutane 536. A facile ring expansion via a 1,3-silyl shift to the terminal methylene group yields 83. Surprisingly, the relative product ratio 82/83 suggests that Afor the forbidden [2 + 2] reaction is slightly smaller than for the stereospecific allowed Diels-Alder path54. 

This rearrangement proceeds via a bicyclic acetal (e.g., 30), and therefore it is a double isomerization involving a ring expansion followed by a ring contraction. It was shown to be most effective using benzamide or pivalamide derivates and the ratio of the ester to the bicyclic acetal was dependent on the reaction conditions used for example, with 5mol% of MeOTf at 130 °C in chlorobenzene, 53% of 29 and 14% of 30 was formed. 

Palladium-catalyzed three-component coupling of dimethylsilacyclobutane, carbon monoxide, and aromatic iodides also yields cyclic silyl enol ethers via a ring/expansion/-insertion process <1996CC1207>. Electron-rich and electron-deficient aromatic iodides are suitable substrates, giving rise to the corresponding cyclic silyl enol ethers in excellent yields (Scheme 50). 

With unsaturated aldehydes as the carbonyl component in the Peterson step, dienes are produced which undergo a double ring expansion via the allylidenecyclopropane rearrangement to give 7,5-fused ring systems such as 77.86... 

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