Carbo-annulation

Extension to carbocyclization of butadiene telomerization using nitromethane as a trapping reagent is reported (Eq. 5.48).72 Palladium-catalyzed carbo-annulation of 1,3-dienes by aryl halides is also reported (Eq. 5.49).73 The nitro group is removed by radical denitration (see Section 7.2), or the nitroalkyl group is transformed into the carbonyl group via the Nef reaction (see Section 6.1). 

As with the nonannulated aminopyrans, general Methods 1-3 have found application in the synthesis of carbo-annulated aminopyrans. 

In summary, the Danheiser reactions are a family of [3 + 2] annulations that take place between allenylsilanes and diverse electrophiles. The products can be carbo-cycles or heterocycles. In all cases, the annulations proceed most efficiently when the allenylsilane has a non-hydrogen substituent on the carbon atom bearing the silicon. This is a consequence of the common mechanistic pathway that proceeds through a vinyl carbocation intermediate. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Aminopyrans, annulated directly with a seven-membered carbo-cycle, are very rare. One, 139, is formed from substituted cyclohepta[c] furan-4-one 140 with UNs 30 (08CFIE136) (Scheme 50). 

B. J. Fitzsimmons and B. Fraser-Reid, Annulated pyranosides as chiral synthons for carbo-cyclic systems. Enantiospecific routes to both (+) and (-) chrysanthemum carboxylic acids from a single progenitor, J. Am. Chem. Soc. 101 6123 (1979). 

Second of all, on certain conditions an appropriate ort/zo-substituent in electronically excited aryl ketones (-R, -OR, -NR2 with R = alkyl) can serve as alternative hydrogen donor, i.e., an hydrogen atom may be abstracted from this ort/zo-substituent rather than from the side chain. In this way, carbo- and heterocycles with an annulated aromatic ring are accessible via Norrish-Yang reaction. (For synthetic applications see Secs. 3.4.2 and 3.4.3.)... 

Abstract Palladium readily catalyzes the cross-coupling of functionally substituted aryl or vinylic halides and alkynes to afford a wide variety of heterocycles and carbocycles in one efficient step. Terminal alkynes presumably initially generate aryl (vinylic) alkynes, which under the reaction conditions are rapidly cyclized by the palladium or copper salts employed in the first step to produce the final product. Internal alkynes apparently react by carbo-palladation of the alkyne and subsequent intramolecular nucleophilic substitution of the palladium moiety to generate the observed products. A variety of other closely related processes have also been reported, including the simultaneous annulation of alkynes and car-... 

Lateral LithiationP Laterally metalated o-toluic acids (eq 48), esters (eq 49), and amides (eq 50) are important intermediates for chain extension and carbo- and hetero-ring annulation. Remote lateral lithiation of 2-methyl-2 -carboxaminobiaryls constitutes a general regiospecific synthesis of 9-phenanthrols (eq 51). ... 

Two reactions formerly used widely in the formation of carbo-cycles have now been successfully extended to the synthesis of heterocyclic systems. Thus the intramolecular cycloaddition of the olefinic alkoxyketenes (36) leads to cyclobutanone annulated heterocycles (37), where better yields are obtained by using the... 

To vary the electronic properties of the carbene carbon atom a number of carbo- and heterocycle-annulated NHCs were prepared and studied. Single pyrido-annulated NHCs (93-96) as well as the doubly pyrido-annulated carbene 97 were obtained by deprotonation of the corresponding azolium salts. NHCs such as the quinone-annulated carbene 98/ the naphtho-annu-lated carbene 99 and related polycyclic compounds have also been obtained from more extended ring systems. 

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