Cycloadditions 2- trimethylsilyloxy-1,3-butadiene

Based on bis-silylated dienes another approach to quinoxaline derivatives such as 80 (Scheme 4.10) was found [97]. Fast [4+2] cycloaddition takes place by treatment of Cgo with 2,3-bis(trimethylsilyloxy)butadiene 98, yielding the acyloin-fused fullerene derivative 100 in good yields (Scheme 4.16). The silylated diene is formed in situ by treatment of 98 at 180 °C in o-dichlorobenzene. Controlled bromination of the intermediate 99 leads to the transient diketone 101, which reacts readily in a one-pot reaction with various o-diaminoarenes to yield the quinoxaline-fused fullerenes 102. 

Molecular electrostatic potentials have been used to explain the regioselectivity exhibited in the Diels-Alder cycloaddition reactions between 1-trimethylsilyloxy-butadiene and the quinones 5-formyl-8-methyl-1,4-naphthoquinone, 5-methoxy-7-methyl-1,4-phenanthrenequinone, and 5,6,7-trimethyl-1,4-phenanthrenequinone.128 The intramolecular Diels-Alder reaction of masked o-benzoquinones (123) with a variety of dienes provides adducts (124) which rearrange to functionalized ris-decal ins (125) with complete stereocontrol of up to five stereocentres. This methodology ... 

A review (37 refs.) on the application of molecular orbital calculations to the stereoselectivity of addition reactions of nitro-enitol derivatives has been published in Japanese. Cycloaddition of 1-acetoxy- or 1-trimethylsilyloxy-butadiene to l,2-dideoxy-l-nitro-D-g/ co-hept-l-enitol tetraacetate gave mainly the cyclohexane derivatives 41. Additions of alcohols, carbon radicals and phosphoiyl... 

There have been few mechanistic studies of Lewis acid-catalyzed cycloaddition reactions with carbonyl compounds. Danishefsky et ah, for example, concluded that the reaction of benzaldehyde 1 with trans-l-methoxy-3-(trimethylsilyloxy)-l,3-di-methyl-1,3-butadiene (Danishefsky s diene) 2 in the presence of BF3 as the catalyst proceeds via a stepwise mechanism, whereas a concerted reaction occurs when ZnCl2 or lanthanides are used as catalysts (Scheme 4.3) [7]. The evidence of a change in the diastereochemistry of the reaction is that trans-3 is the major cycloaddition product in the Bp3-catalyzed reaction, whereas cis-3 is the major product in, for example, the ZnCl2-catalyzed reaction - the latter resulting from exo addition (Scheme 4.3). 

In contrast with the metal-free cycloaddition again, the efficiency of metal mediated cycloaddition reactions is relatively insensitive to the electronic nature of the reactants. This has been nicely demonstrated by Rigby and colleagues305 who treated complex 494 with a 1 1 mixture of methyl sorbate (502) and 1-trimethylsilyloxy-l,3-butadiene (50). The reaction proceeded in 90% yield and afforded 503 and 504 in a 46 54 ratio (equation 146). 

Diels-Alder reactions with aldehydes.2 This catalyst is superior to zinc chloride3 for promoting [4 + 2]cycloaddition of aldehydes with l-alkoxy-3-trimethylsilyloxy-1,3-butadienes to form 2,3-dihydro-4//-pyrane-4-ones. The catalyst prepared from bornyl alcohol is somewhat more effective than similar catalysts from simple alcohols, and may be of value for asymmetric induction. 

Very recently, chiral tricarbonylchromium complexes have been introduced as novel chiral auxiliaries for aza Diels-Alder reactions [192, 193]. Using the brominated imine 3-8, Kiindig s group was successful in efficiently generating enantiopure polycyclic compounds such as 3-10 by cycloaddition of 3-8 to l-methoxy-3-trimethylsilyloxy-l,3-butadiene (Danishefsky s diene), subsequent radical cyclisation of the cycloadduct 3-9 and oxidative metal removal from 3-11 (Fig. 3-3). 

In a similar fashion, Achmatowicz et al. have studied the cycloaddition of 1-acetoxy- and l-trimethylsilyloxy-3-methyl-l,3-butadiene with diethyl oxoma-lonate and isopropyl glyoxylate under thermal and high pressure conditions [542],... 

One of the most popular dioxygenated dienes is l-methoxy-3-trimethylsilyloxy-1,3-butadiene (109), the so-called Danishefsky diene (Scheme 30). The elegant application of this and related polyoxygen-ated dienes in synthesis was reviewed in 1981. ° [4 + 2] Cycloadditions of diene (109), wifo typical electron-poor dienophiles, display increased reactivity and legiochemical control owing to die synergism of die two oxygen atoms. Acidic hydrolysis of the initial cycloadducts readily affords cyclohexenones, phenols or cyclohexadienones (c/. Action 4.1.2, Schemes 12, 17,18 and 21). A pertinent example is the... 

A select set of useful [4 + 2] cycloaddition reactions of simple 2-aza-1,3-butadienes have been detailed (Table 11), " and in most instances the 2-azadienes employed are substituted with strong electron-donating substituents responsible for enhancing its Diels-Alder reactivity toward electron-deficient dienophiles. The ease with which the l,3-bis(t-butyldimethylsilyloxy)-2-aza-1,3-butadienes may be prepared from imides, and the demonstrated facility with which they participate in HOMOdiene-controlled [4 + 2] cycloaddition reactions, may prove exceptionally useful their use constitutes the most general approach to implementing the 477 participation of 2-aza-1,3-butadienes (Scheme 12). With the recent introduction of a convenient preparation of 3-trimethylsilyloxy-2-aza-1,3-butadienes, their comparable synthetic utility may be anticipated. ... 

The 1-azadiene and the dienophilic character of 2-alkenyl-5,6-dihydro-4//-l,3,4-oxadiazines (150 Section 6.17.6.2) have been explored (Scheme 21) <93TL457>. Attempts to effect cycloaddition with DMAD, dimethyl fumarate, and acrylonitrile failed, whereas with maleic anhydride and A-methylmaleimide unexpected noncyclic adducts (e.g. 151) are obtained. The alkenyl oxadiazines also fail to give cycloadducts with l-methoxy-3-trimethylsilyloxy-l,3-butadiene with or without the presence of a Lewis acid catalyst. However, with methyl penta-2,4-dienoate mixtures of diastereo-isomeric products (e.g. 152) are formed, albeit slowly, by [4 + 2] cycloaddition at the alkenyl side chain, rather than at the C2—N3 imine bond. 

Kametani and co-workers have demonstrated what may prove to be the most convenient and effective approach to the utilization of l-aza-1,3-butadienes for intramolecular [4 + 2] cycloadditions.37 Treatment of a,j3-unsaturated enamides with trimethylsilylchloride-zinc chloride provides in situ generation of 2-trimethylsilyloxy-l-aza-1,3-butadienes which are capable of effective intramolecular [4 + 2] cycloaddition (Table 9-II, entries 12-14). Selected conditions have permitted this in situ generation and subsequent intramolecular Diels-Alder cycloaddition of 2-trimethy-lsilyloxy-l-aza-1,3-butadienes to be conducted at 25°C,37b and these observations have found application in the total syntheses of ( )-epilupinine and ( )-tylophorine [Eqs. (14) and (15)].37c,b... 

The two diastereoisomeric piperidone acyclo C-nudeosides 847 and 848 were formed by [4 + 2] cycloaddition of the sugar azomethine derivative 846 to 2-trimethylsilyloxy-4-methoxy-l,3-butadiene (95T2969) (Scheme 238). 

Bis(methylthio)pyrimidines 40 have been obtained upon reaction of ketones with triflic anhydride (94MIS59]. Cycloaddition reactions of l,3-diaza-l,3-butadienes with haloketenes have been employed for the synthesis of pyrimidones [94T(50)7579). Reaction of 2-trimethylsilyloxy- and 2-trimethyI-silylthio-l,3-diazabutadienes (41) with enamines derived from aliphatic aldehydes (42) leads regio- and stereoselectiveiy to tetrahydropyri-midin-2(l -ones and thiones (43). This reaction type was also used for preparing quinazolines [94H(37)1109],... 

The application of Lewis acids, such as the europium salt Eu(fod)3, in [4+2] cycloadditions of carbonyl compounds (e.g. glyoxylates and aldehydes) with 1-methoxy-3-trimethylsilyloxy-l,3-butadiene (Danishefsky s diene) has been intensively examined by Danishefsky et al. and employed in natural product synthesis. ... 

Danishefsky and coworkers have used the racemic P-phenylsulfinyl-a,P-unsaturated ketone (160) as an a,P-ethynyl carbonyl synthetic equivalent and found that the phenylsulflnyl group did not compete with the carbonyl group in determining the regioselectivity of cycloaddition with the l-methoxy-3-trimethylsilyloxy-1,3-butadiene (161), now known as Danishefsky s diene [140] (Scheme 5.54). Loss of methanol and phenyl sulfenic acid from the initial cycloadduct (162) gave the aromatic product (163). 

Other dienes, such as cyclohexadiene, anthracene and cyclopentadiene have also been reacted with a-oxo sulfines. The cycloaddition of a-oxo sulfines with 2-trimethylsilyloxy-1,3-butadienes afford thiacyclohexane-3-one S-oxides after hydrolysis . ... 

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