Semicyclic diene

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... 

This appears to be related to the availability of allylic hydrogens capable of approaching a position orthogonal to the olefinic plane, thus favoring the ene reaction [40]. Attempts to control the diastereoselectivity with a hydroxy group next to Cl lead to moderate facial selectivity (Sch. 29) [lOd], and similar results have been found in semicyclic dienes with the hydroxy group next to C2 [40] of the diene moiety. 

A semicyclic diene one of the double bonds forms part of a ring and the other is exocyclic, or outside the ring. When only one of of the two sp hybridized carbons of a double bond is a part of the ring under... 

In the case of semicyclic dienes where one of the ethylenic linkages forjus part of a ring and the other is exocyclic, the absorption bands a e fotmxl at lower wavelengths (around 235 m/x) than cyclic dienes. The l lion spectra of cyclic dienes have been thoroughly studied in the stolid ana- i... 

Carbohydrate-Derived Dienes as a Model for Semicyclic Dienes. W e... 

The 7C-facial selectivity in dienes, 2, 3 and 38, 39, anti to the allylic alkoxy group, is the same as the one found by Overman and Hehre in semicyclic dienes (47). 

Semicyclic Dienes with One or Two Allylic Stereogenic Centers. The... 

Semicyclic Dienes with One Allylic Stereogenic Substituent at Position 2 in the Diene Moiety. From the above mentioned reasoning, the question arose about if any of the stereodirecting effect would be motivated by the additional allylic substituent at position 2 and, furthermore, if a single allylic substituent at position 2 in the diene system (C in Figure 15) would have some effect in the stereochemical outcome of the cycloaddition reaction. 

The first successful natural product synthesis incorporating a DTDA reaction sequence was published by Sherburn and coworkers in 2008, and constitutes a formal synthesis of triptolide (165) (Scheme 12.34) [46]. The carbon framework of this natural product was rapidly assembled from silyl protected [3]dendralene 161, beginning with a DA reaction between the free alcohol and methyl acrylate. In situ lactonization provided semicyclic diene 162, which was subjected to a high-pressure DA reaction with quinone 163. Tetracycle 164 was functionalized further to intercept an intermediate from Berchtold s 1982 total synthesis of triptolide [47]. 

The first examples of a consecutive radical 5 -exo-/dig-5-exo-dig cyclization of 1,5-diynes have been accomplished by the same researchers [43]. These authors were able to show that their cycloisomerization procedure provides access to strained semicyclic, conjugated dienes with a functionalized dioxatriquinane framework which occurs in the aglycones of steroidal cardiac glycosides, such as isogenine (3-96) [44] and C-norcardanolide (3-97) (Scheme 3.24) [45]. 

In allylidenecyclopropanes, the semicyclic double bond can be part of a diene system capable of undergoing Diels-Alder reactions with appropriate dienophiles such as tetracyanoethene, maleic anhydride, benzoquinone, dimethyl (E)- and (Z)-but-2-enedioate and acetylenedicarb-oxylate. ... 

Semicyclic chiral dienes 44 containing a sulfinyl group react with jV-phenylmaleimide (2) in a complete anti-facial selectivity. The observed total selectivity (d.r. 100 0) is caused by the electrostatic interaction between the carbonyl oxygen and the sulfinyl group during the transition state15. 

Scheme 2.27 Relay enyne RCM/CM reaction to give the semicyclic 1,3-diene 70.

KR of racemic semicyclic 1,3-dienes was observed in their Diels-Alder reaction with enantiomerically pure (S)-(2-p-tolylsulfinyl)-l,4-naphthoquinone (4-)-58 [58]. As an example, [4-t-2] cycloaddition of (+)-58 with ( )-59 (1 equiv) afforded (3-)-anti-60 as the sole stereoisomer, showing >94% ee. The recovered unreacted diene (S)-59 showed 50% ee. The initial cycloaddition thus occurred in a highly endo and jt-facial diastereoselective manner with an efficient resolution of the diene partner. 

Cyclo [3] dendralenes contain both a semicyclic and acyclic diene site, and, in principle, either of the two tricyclic frameworks 92 or 94 can be accessed through alternative DTDA sequences (Scheme 12.19). 

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