High-pressure Diels-Alder

Table 5.1 High pressure Diels-Alder reactions of (E)-l-acetoxy- (18a) and (E)-l-methoxy butadiene (18b) with acrylic and crotonic dienophiles...

Table 5.2 High pressure Diels-Alder reactions of diene 77 with dienophiles 78...

Enantiomers (M)- and (P)-helicenebisquinones [32] 93 have been synthesized by high pressure Diels-Alder reaction of homochiral (+)-(2-p-tolylsulfo-nyl)-l,4-benzoquinone (94) in excess with dienes 95 and 96 prepared from the common precursor 97 (Scheme 5.9). The approach is based on the tandem [4 + 2] cycloaddition/pyrolitic sulfoxide elimination as a general one-pot strategy to enantiomerically enriched polycyclic dihydroquinones. Whereas the formation of (M)-helicene is explained by the endo approach of the arylethene toward the less encumbered face of the quinone, the formation of its enantiomeric (P)-form can be the result of an unfavourable interaction between the OMe group of approaching arylethene and the sulfinyl oxygen of 94. 

An interesting phenomenon has been observed in the high pressure Diels-Alder reactions of the l-oxa[4.4.4]propella-5,7-diene (117) with 1,4-naphthoquinone, maleic anhydride and N-phenylmaleimide, where the diene 117 undergoes a rearrangement to the diene isomer 118 which, although thermodynamically less favored, exhibits a greater reactivity [40]. The reactivities of the three dienophiles differed since maleic anhydride and N-phenylmaleimide reacted only in the presence of diisopropylethylamine (DIEA) and camphorsulfonic acid (CSA), respectively (Scheme 5.15). The distribution of the adduct pairs shows that the oxygen atom does not exert a consistent oriental dominance on TT-facial selectivity. 

The high pressure Diels-Alder reactions of quinone-mono-ketals [80]... 

Synthesis and high-pressure Diels-Alder cycloadditions of 6-methoxycarbonyl-3-oxo-2-azabicyclo[2.2.0]hex-5-ene [86]... 

High-pressure Diels-Alder reaction of 1-methyl-2-(1H)-pyridones having a phenyl group with N-phenylmaleimide [94]... 

Minuti L. and Taticchi A. Multiple and High Pressure Diels-Alder Reactions in... 

Polysubstituted pyridines 312 can be prepared by a sequence involving an intramolecular thermal or high-pressure Diels-Alder cycloaddition of oximino dienophile 310,... 

The related 5-trifluoromethyl-l,3-dioxin-4-ones underwent high pressure Diels-Alder reactions with Danishefsky s diene (Eq. 114), and [2 + 2] photocycloadditions with alkenes [305]. The former reaction failed entirely when attempted in the absence of the fluorine atoms. 

Drawn from these examples it is apparent that controlling the chemose-lectivity in inter-intermolecular Heck-Diels-Alder reactions of two different alkenes can be tedious if the alkenes show comparable reactivities. Nevertheless, the stepwise approach was realized in several other cases. In a synthesis of a derivative of cephalostatin 1 containing a central benzene instead of the pyrazine ring, Winterfeldt et al. linked two steroidal systems by a Heck coupling and subsequently performed high pressure Diels-Alder reactions of the conjugated diene with electron-deficient alkynes [34], Another example, reported by Hayashi et al., involves a selective Heck reaction of a bromoglu-cal with ethylene or acrylic acid derivatives followed by cycloadditions with maleic anhydride or N-phenylmaleimide [35]. 

Reaction of dienoaldehyde with the stabilized Wittig reagent(s) provided a triene 88, which upon high-pressure Diels-Alder cyclization furnished bicyclo[4.3.0]nonene [78]. Alternatively the aldehyde S7-E was converted into the methyl ester 89, which reacted with the dimethyl methylphosphonate anion affording the phosphonate 90. Reaction of the latter with aldehydes under mild PTC conditions yielded a (regioisomeric to the previous one) triene 91, which cyclized spontaneously to bicyclo[4.4.0]decene (O Scheme 37) [79]. 

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