Alkenes intramolecular

The reaction course is shown in Scheme 4. Enyne 12 reacts with 2 to give vinyl carbene complex 17, which is in a state of equilibrium with vinyl ketene complex 21. [2+2] Cycloaddition of the ketene moiety and alkene part in 21 gives cyclob-utanone 22. On the other hand, the vinyl carbene complex 17 reacts with the alkene intramolecularly to produce metalacyclobutane 18. From metalacyclob-utane 18, reductive elimination occurs to give cyclopropane derivative 23. Ret-... 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

Although it probably did not involve a Heck reaction per se, Balme and co-workers employed an interesting tandem reaction in their construction of A 2) capnellene (147) (Scheme 6-26) [54J. Presumably vinyl iodide 144 undergoes initial oxidative addition with the palladium(O) catalyst to furnish a cr-alkenylpalladium(n) intermediate that is complexed to the pendant alkene. Intramolecular addition of the soft malonate nucleophile to this complex, from the opposite face, followed by reductive elimination, then provides tricycle... 

Considerably better yields are obtained when the alkene is significantly strained, as in the synthesis of [3]peristylane (5). Note in this case that flash-vacuum pyrolysis is the method of choice for the final step. The intramolecular addition step in the synthesis of ( —)-cyclo-copacamphene (8) and in a very similar synthesis of (+ )-cyclosativene presumably also gain assistance from ring strain. It should be noted that in these last two syntheses, the diazoalkane-alkene intramolecular cycloaddition succeeded where attempted carbene-alkene cycloadditions had failed. 

This arene-alkene intramolecular meta photocycloaddition was shown to provide the basis for an effective and general approach to polyquinane natural products, such as (+ )-cedrene or ( + )-isocomene. ... 

When the starting material carries a but-3-enyl side chain in a suitable position, the intermediate carbon radical adds to the alkene intramolecularly to furnish bicyclic ketones in good yield. 

Photoaddition of the ynone (47) to 1,1-diethoxyethene gave the oxetan (48) (50%), by a conventional (2+2)-cycloaddition route, as well as the furan derivative (49) (18%). The formation of this latter product arises by way of the carbene (50), itself a product of (3- -2)-addition of the ynone to the alkene. Intramolecular trapping of the carbene... 

When TMM equivalents are tethered to alkenes, intramolecular cyclization generates bicyclic methylene-cyclopentane derivatives. Cyclization of 394, for example, led to a 51% yield of 395 along with 18% of the uncyclized triene, 396. 35 jhe cyclization reaction proceeded with excellent diastereoselectivity, as shown. Trost has also examined the attachment of chiral auxiliaries to the alkene partner to give asymmetric induction in reactions with TMM equivalents.231... 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Generation of 1-Azadienes. 0-Silyloximes have also been used to generate 1-azadienes for [4 + 2] cycloadditions. Thus an 0-silyloxime of an a, -unsaturated aldehyde can be treated with an acid chloride or chloroformate in the presence of chlorotri-methylsilane and aluminum chloride to give an a-cyanohydrox-amic acid derivative, which upon mild thermolysis forms an aza-diene (eq 2). These azadienes undergo efficient intermolecular [4 + 2] cycloadditions or, with a tethered alkene, intramolecular cycloadditions. 

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