Diazoketones silyl

Whereas metal-catalyzed decomposition of simple diazoketones in the presence of ketene acetals yields dihydrofurans 121,124,134), cyclopropanes usually result from reaction with enol ethers, enol acetates and silyl enol ethers, just as with unactivated alkenes 13). l-Acyl-2-alkoxycyclopropanes were thus obtained by copper-catalyzed reactions between diazoacetone and enol ethers 79 105,135), enol acetates 79,135 and... 

Silyl-substituted diazoketones 29 cycloadd with aryl isocyanates to form 1,2,3-triazoles 194 (252) (Scheme 8.44). This reaction, which resembles the formation of 5-hydroxy-l,2,3-triazoles 190 in Scheme 8.43, has no analogy with other diazocarbonyl compounds. The beneficial effect of the silyl group in 29 can be seen from the fact that related diazomethyl-ketones do not react with phenyl isocyanate at 70 °C (252). Although the exact mechanistic details are unknown, one can speculate that the 2-siloxy-1-diazo-1-alkene isomer 30 [rather than 29 (see Section 8.1)] is involved in the cycloaddition step. With acyl isocyanates, diazoketones 29 cycloadd to give 5-acylamino-l,2,3-thiadiazoles 195 by addition across the C=S bond (252), in analogy with the behavior of diazomethyl-ketones and diazoacetates (5). 

In this context, it is worth mentioning that there is only one other, clear-cut example for the simultaneous occurrence of the acyl(silyl)carbene-to-acylsilene and the acylcarbene-to-silylketene rearrangement of an acylcarbene bearing a Si—Si substituent. Carbene 57, generated by photolysis of diazoketone 56 in benzene, isomerized to both 58 and 59 in about equal amounts44. While the acylsilene cyclized to 1,2-silaoxetene 60, the ketene was isolated and structurally characterized by X-ray diffraction analysis of the derived... 

Photolysis of disilanyl a-diazoketones 497 leads through the sequence involving silyl carbene formation followed by rearrangement to acyl sil-enes and their facile cyclization to give l-oxa-2-sila-3-cyclobutenes 498 (84JA1486 88CC72 90CB589). 

Two short syntheses of racemic ipalbidine ( )-(842) are shown in Scheme 109. The synthesis by Jefford et al. commenced with conjugate addition between pyrrole and Ae atropate ester 849 followed by homologation of the acid 850 with diazomethane and rhodium-induced intramolecular carbene cyclization of the resulting diazoketone 851 (574). The bicyclic product 852 was converted into ( )-842 in a further four steps. The approach taken by Danishefsky and Vogel centered on acid-catalyzed cyclocondensation between the silyl ketene acetal 853 and A -pyrroline (854) to give indolizidinone 855 (575). Reduction of the lactam and cleavage of the aryl ether completed the synthesis of ( )-842. 

The total synthesis of the isoprostane ( )-8-epi-PGF2a ether is accomplished via cyclization of diazo keto silyl derivative 200. Aldol condensation of diazoketone 198 with decadienal (199) in the presence of KHMDS followed by addition of LiBr provides an intermediate P-hydroxy adduct 201 which is not isolated but is immediately silylated to give silyl derivative 200.86 Diazo hydroxy ketones 201 and 203 are silylated with ClSi(Et)3 and chloro dimethyl-Mributylsilane to afford silylated diazoketones 202 and 204, respectively.86... 

Wolff rearrangement. Silyl ketenes are readily formed from a-silyl-a-diazoketones on exposure to rhodium(II) octanoate. ... 

The continuing development of solid-phase synthesis and combinatorial chemistry has led to solid-phase oxazole syntheses with a minimum of purification. Iso and co-workers generated ot-(trimethylsilyl)diazoketones on a Wang resin and employed rhodium-catalyzed diazo transfer methodology to prepare oxazoles (Scheme 1.47). Reaction of the resin-bound benzoyl chloride 169 with (trimethyl-silyl)diazomethane gave the corresponding a-(trimethylsilyl)diazoketone 170 in excellent yield. Treatment of 170 with an aryl nitrile in the presence of catalytic Rh2(OAc)4 then furnished a resin-bound 2,5-diaryl-4-(trimethylsilyl)oxazole 171. 

Trimethylsilyldiazomethane smoothly reacts with (trialkyl-silyl) vinylketenes, ultimately prepared from diazoketones, in a net [4+1] annulation process to afford 2-trialkylsilylcyclopentenones in good to excellent yields (eq 68). The products are readily desi-lylated by treatment with methanesulfonic acid in methanol. An analogous reaction with (trialkylsilyl)arylketenes gives trialkyl-silyl-2-indanone derivatives (eq 69). ... 

Wolff rearrangement has also been applied for the synthesis of silylketenes 25 from silyl diazoketones 24. The ketenes 25 upon further reaction with BnNH2 afforded the corresponding a-silyl benzylamides 26 in good yields. 

When acyl(disilanyl)carbenes 48 are generated by photolysis of diazoketones 47 in benzene solution, the acyl(silyl)carbene-to-acylsilene rearrangement appears to be the exclusive consequence. The further fate of the acylsilenes 49 depends on the nature of substituent R (equation 11). When R is bulky (1-adamantyl (1-Ad), t-Bu), 1,4-cyclization yields the 1,2-silaoxetanes 50, which easily decompose to an alkyne and dimethylsilanone... 

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