Silyloxydienes

Vinylogous aldol addition of heterocyclic silyloxydienes in organic synthesis 99SL1333. 

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

A novel and versatile method for preparing polymer-supported reactive dienes was recently developed by Smith [26]. PS-DES (polystyrene diethyl-silane) resin 28 treated with trifluoromethanesulfonic acid was converted to a polymer-supported silyl triflate 29 and then functionalized with enolizable a,jS-unsaturated aldehydes and ketones to form silyloxydienes 30 and 31 (Scheme 4.4). These reactive dienes were then trapped with dienophiles and the Diels Alder adducts were electrophilically cleaved with a solution of TFA. 

A polymer-supported silyl triflate and subsequent functionalization synthesis and solid-phase Diels-Alder reactions of silyloxydienes [25]... 

Asymmetric Diels-Alder reactions. Diels-Alder reactions of aldehydes with silyloxydienes catalyzed by this chiral organoaluminum compound can proceed with high enantioselectivity. Thus reaction of benzaldehyde with the silyloxydiene 2 at... 

Scheme 60 Diels-Alder reaction of ethyl vinyl ketone with silyloxydienes...

Cyclocondensation of aldehydes with silyloxydienes (12, 312). The stereoselectivity of this reaction depends not only on the catalyst but also on the solvent as summarized in equation (I) for reactions with l-methoxy-2-methyl-3-trialkylsi-lyloxy-l,3-pentadienes la and lb.8... 

Diels-Alder reactions.1 This highly substituted silyloxydiene is comparable to 1-methoxy-3-trimethylsilyloxy-l,3-butadiene (6, 370 9, 303-304) in reactivity in Diels-Alder reactions. 

The intermolecular Diels-Alder reaction between the dibromoenone (111) and dienes (112) provides access to bicyclo[5.4.0]undecane systems (113) that are common core structures of many natural products (Scheme 32).118 The alio-threonine-derived O-(/ -biphenyl carbonyl oxy)-/i-phenyloxazaborolidi none catalyses the enan-tioselective Diels-Alder reaction of acyclic enones with dienes.119 The reversal of facial selectivity in the Diels-Alder cycloaddition of a semicyclic diene with a bro-moenone was induced by the presence of the bromo substituent in the dienophile.120 Mixed Lewis acid catalyst (AlBr3/AIMe3) catalyses the Diels-Alder reaction of hindered silyloxydienes with substituted enones to produce highly substituted cyclohexenes.121 Chiral /V-enoyl sultams have been used as chiral auxiliaries in the asymmetric Diels-Alder reactions with cyclopentadiene.122... 

Similarly, the reaction for formyl- and keto-chromones with silyloxydienes produced benzopyryliums which initiated a sequence that resulted in the formation of a 2,4-dihydroxybenzoquinone such as 112 (Equation 7) <2006EJO3638>. 

Finally, a thia Diels-Alder reaction representing a less common cycloaddition type in natural product synthesis shall be discussed. Thus, Vedejs et al. have included such a cycloaddition into an elegant strategy aimed at the synthesis of macrocyclic [ll]-cytochalasans such as zygosporin E 7-76 [536-538]. Thus, release of the thioaldehyde 7-73 from its phenacyl sulfide precursor in the presence of the silyloxydiene 7-74 yielded 7-75 as 2 1 mixture with its C20 epimer. Fortunately, equilibration of this mixture raised the ratio up to 10 1. Several subsequent steps yielded the tetracyclic intermediate 7-77 cleavage of its thioether moiety then liberated the 11-membered macrocycle present e.g. in zygosporin E 7-76 (Fig. 7-16). 

Selection of Lewis acids by their principal properties for synthetic exploration has been reported for syntheses of benzamides [59], of silyloxydienes [33], and of indoles [60]. 

Silyloxydienes in rego and stereo controlled Dieis-Alder and heiero Diels-Alder reactions... 

Danishefsky has extended this methodology for the synthesis of ( )-fucose and( )-daunosamine(85JA1269). Reaction of silyloxydiene 195 with acetaldehyde (197) in the presence of anhydrous zinc chloride gave 90% of a 3.3 1... 

Two homo Diels-Alder reactions and a hetero Diels-Alder reaction, each using a silyloxydiene, have been used in a total synthesis of vinemycinone methyl ester (236) by Danishefsky (84JA2453 85JA1285). Ketoaldehyde 231, prepared by two Diels-Alder cycloadditions, reacted with diene 230 in chloroform in the presence of Eu (fod)j to produce the silyl enol ether... 

Easily prepared from glycols, enones have been investigated as dienophiles. They react with butadiene under Lewis acid catalysis to form chiral cyclohexenes used in the synthesis of compactin analogs [353]. Levoglucosenone has been used in a Diels-Alder reaction with acetoxy-butadiene to construct a part of the indole alkaloid reserpine [354], and in synthetic studies toward tetrodotoxin [355]. Analogs of the anthracycline rhodomycinone have been similarly prepared [356]. [4 + 2]-Cycloaddition of the same enone with silyloxydiene allowed the creation of the fused ring system present in actinobolin [357]. 

Fig.12.10 Residual plots in the silyloxydiene study faj Normal probability plot (b) Plot of residuals against predicted response.

Saturated O-heterocycles with P-containing substituents 82UK412. Silyloxydienes in total synthesis of natural O-heterocycles 81ACR400. Silylated N,O-heterocycles, synthesis of 80MI20. 

Danishefsky has exploited his widely utilized silyloxydiene chemistry to complete a formal total synthesis of 90 (Scheme 1.20). By employing the appropriate oxidation levels for both the diene and dienophile, a resorcinyl ester possessing the required differentiation of the phenolic groups was obtained without further oxidative manipulation. To this end, the dianion of propiolic acid was alkylated with l-bromo-7-octene to give acid 98 in 68% yield. Further alkylation with methyl iodide then gave the ester 99. A Diels-Alder reaction with diene 100, a derivative of methyl acetoacetate, and alkyne 99 then furnished an initial phenolic intermediate which was protected as the benzyl ether to afford... 

Cycloaddition of the endocyclic 2-silyloxydiene (106) to methyl vinyl ketone gave the bicy-clo[2.2.2]octenes (107) with a silyloxy-diiected regiochemistiy. The major endo product was then transformed into the tricyclic sesquiterpene ( )-seychellene (108) (Scheme 29). ... 

Among other carboxylic acid derivatives, the a,p-unsaturated lactams 1.92 proposed by Meyers and cowotkers [327] give useful results. No catalysis is needed in cydoadditions with silyloxydienes reactions with 2,3-dimethylbutadiene (R = R I Me) (F %... 

Silyloxydienes in regio- and stereo-controlled Diels-Alder and hetero Diels-Alder reactions (see 1st edition). 

The above reaction type was also applied to the acetoxyazetidinone (81).Heating of (81) with diene (82) and ZnCh in acetonitrile at reflux gives co. 60% of (formal) cycloadduct (83) as a single trans stereoisomer (equation 40). From (83) the carbapenam skeleton was readily prepared in three steps. The reaction was repeated for silyloxydiene (84) in order to gain access to the more potent l -methyl analog of thienamycin. In refluxing acetonitrile the monocycle (86) is formed as the major product (equation 41). 

Stereoselective oxycarborative addition is also achieved in cycloaddition and cyclooligomeriza-tion reactions. Thus, hetero-Diels-Alder reactions of dienes and aldehydes are not only catalyzed by main group Lewis acids, but also by transition metal complexes 10°. Tris[3-(heptafluoropropyl-hydroxymethylene)-( + )-camphorato]europium [( + )-Eu(hfc)3] and similar vanadium complexes have been used as the chiral catalyst in [4 + 2] cycloadditions of various achiral and chiral dienes to aldehydes63 67-101. With achiral silyloxydienes only moderate asymmetric inductions are observed, however, with chirally modified dienes, high double diastereoselectivities are achieved. Thus, the reaction of benzaldehyde with 3-terf-butyldimethylsilyloxy-l-(/-8-phenvl-menthoxy)-l.3-butadiene (1) gives (2/ .6/ )-4-wf-bntyldimethylsilyloxy-5,6-dihydro-6-phenyl-2-[(17 ,3/ ,45 )-8-phenylmenthoxy]-2f/-pyran (2) in 95% yield with a diasteieoselectivity of 25 1 ss. After crystallization and hydrolysis with trifluoroacetic acid, optically pure (2/ )-2,3-di-hydro-2-phenyl-4-(4//)-pyranone (3) is obtained in 87% yield. 

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