Intramolecular cycloadditions electrocyclizations

The photocycloaddition of aliphatic and aromatic aldehydes with 2,4,5-trimethyloxazole (131) gave bicyclic oxetanes 132 in almost quantitative yields hydrolitic cleavage led selectively to erytro a-amino-P-hydroxy methyl ketones 133 <00CC589>. The oxazolium salt 134 was converted to the azomethine ylide 136 via electrocyclic ring opening of the oxazoline 135. Intramolecular cycloaddition afforded 137 in 66% overall yield which was transformed into the aziridinomitosene derivative 138 . 

Apart from some special cases, the ring-closure step in the course of the synthetic pathway is often condensation reaction and, thus, these transformations can be considered as cyclocondensations . Reactions following a few other mechanisms like cycloaddition, electrocyclization, and intramolecular nucleophilic addition of electron pair of an atom to a multiple bond also occur relatively often. Because of the fairly large complexity of the ring systems belonging to this chapter, the treatment according to the heteroatomic combinations seemed the most straightforward. Thus, the fused... 

An electrocyclic reaction is defined as a concerted closure of a ring starting from a conjugated open system in which a k orbital is converted into a o orbital. It is therefore an example of intramolecular cycloaddition or retroaddition. 

Synthesis of the dibenzofuran (27) by irradiation of grisa-3, 5 -diene-2, 3 -dione (28) is believed to involve electrocyclic ring opening followed by intramolecular cycloaddition to the ketene and elimination of carbon dioxide, as shown in Scheme 2.24 Analogous photocyclizations are responsible for the photochromism exhibited by heterocyclic fulgides such as ( )-a-3-furyl-ethy idene(isopropylidene)succinic anhydride (29), which on irradiation... 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

For the synthesis of estradiol methyl ether 4-319, the cyclobutene derivative 4-317 was heated to give the orthoquinonedimethane 4-318 which cydized in an intramolecular Diels-Alder reaction [109]. The thermally permitted, conrotatory electrocyclic ring-opening of benzocyclobutenes [110] with subsequent intramolecular cycloaddition also allowed the formation of numerous complex frameworks (Scheme 4.70). 

Further support for a photochemical triplet mechanism in a rather different system is that the compound 17 gives a vinylcyclopentcnone (by way of intramolecular cycloaddition followed by electrocyclic ring-opening) only on triplet sensitization (equation 45) . 

In the presence of very reactive dienes, such as o-quinodimethanes, even electron-rich dienophiles, such as oxime ethers, can be reacted, as exemplified in Ae intramolecular cycloaddition of (143 Scheme 65), which is created in situ by an initial (2 + 2 -t- 2] cycloaddition reaction of (140) with (141) and subsequent electrocyclic ring opening of the resulting benzocyclobutene (142). This strategy has been applied recently to a novel isoquinoline synthesis. 

Altenbach used the Michael addition of sodium methyl malonate to allene (206) for a dia-stereoselective spiroannulation to a steroid (equation 72). Or, in imaginative work by Okamura, allenyl sulfoxides were transformed into enantiomerically pure hydrocarbons by pericyclic reactions like electrocyclic ring closure (equation 73) or intramolecular cycloaddition (equation 74). Note that the starting materials (propargylic alcohols) are readily accessible as single enantiomers. 

A quite different product (152) is formed in 70% yield by photolysis of diphenyl derivative (150). Formation of this compound is explained as a result of an electrocyclic conrotatory ring opening leading to (151) and subsequent Diels-Alder-like intramolecular cycloaddition of a carbonyl group to the aza diene moiety. The possibility of this unusual Diels-Alder-like reaction is explained by a favorable position of the CO group in (151) (Scheme 23) <86J0C3123). 

Harrowven and coworkers disclosed a short synthesis of (-)-elisapterosin B (60) and (-)-colombiasin (61) combining the Moore rearrangement and intramolecular cycloaddition reactions (Scheme 8.20) [27b]. The bicyclic quinone 126 was generated from 125 via a domino 4 i-e conrotatory ring opening 125 126/6x-e disrotatory electrocyclization... 

---