Diels-Alder reactions ultrasound promoted

Diels-Alder reactions. Ultrasound can promote some Diels-Alder reactions with o-quinones, and also improve regioselectivity. Highest yields of adducts are obtained in the absence of a solvent. This reaction provides a route to some pigments of Chinese sage responsible for the biological activity of Dan Shen. An example is the synthesis of 3, the acetonide of tanshindiol B. Thus the thermal reaction of the diene 1 with the quinone 2 gives 3 and 4 in the ratio 1 1. Sonication improves both the regibselectivity and the yield. 

Catalysts such as Fe(BuEtCHC02)3 have been d eloped that are effective for the heteroatom Diels-Alder reaction. Indium trichloride (InCls) is a good catalyst for imino-Diels-Alder reactions. Hetero-Diels-Alder reactions involving carbonyls have been done in water. Ultrasound has been used to promote the Diels-Alder reactions of 1-azadienes. ... 

Ultrasound was reported to promote Diels-Alder reactions as well as improve their regioselectivity similarly to high pressure, but much more conveniently (Scheme 56, Table 7). Comparison of entries 2/3 indicates the similarity of the yields and product ratios (242)/(243), obtained at 10 kbar (entry 2) and on sonication (entry 3). The latter were attributed to the implosion of the ultrasound-induced cavities which locally generate high pressures and temperatures (up to 1 kbar and 5000 K). Deacetalization of the major aromatized regioisomer (242) gave the natural product tanshindiol B. 

Table 7 Ultrasound Promoted Diels-Alder Reactions Influence on Yield and Regiochemistry (Scheme 56)...

Dihydroxytanshinone was obtained by the ultrasound-promoted Diels-Alder reaction between a benzofurandione and a vinylcyclohexene, followed by oxidation and desilylation as illustrated in the following scheme <04T1665>. A fully functionalized benzofuran was also utilized as a major building block in the total synthesis of kendomycin <04JA14720>. Benzofuran-based tetraols were also employed as key fragments in the synthesis of some helical molecules <04H(63)137>. 

A variety of other reaction conditions promote the Diels-Alder reaction/ including high pressure, ultrasound, electron transfer, polar... 

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