Results from the Cyclizations

We selected a series of rhodium(II) carboxylates, rhodium(II) carboxamidate [5d] (Doyle catalysts 42h, 42i, 42j), and the bridged rhodium(II) carboxylate (Lahuerta catalyst) 42g, as representatives of the various rhodium(II) catalysts generally utilized. Most of the carboxylate and Doyle catalysts were commercially available and were purified by silica gel chromatography prior to use. The Lahuerta catalyst was prepared according to the literature procedure [23]. 

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The vast majority of syntheses of pyrido[2,3-t/]- and pyrido[3,4-t/]-pyridazines fall into this category, resulting from the cyclization of various o-substituted pyridine derivatives (2,3- or 3,4-, respectively) with hydrazine or its congeners. 

A 1,lO,l9,28-Tetraaza-2,3-naphthalocyanine 3 results from the cyclization of anhydride 2. 

The Bischler-Napieralski reaction is one of the traditional methods for isoquinoline synthesis, and has been applied to the preparation of fused quinolizidine systems. One simple example is the transformation of compound 246 into a 9 1 mixture of diastereomers 247 and 248 by treatment with phosphorus oxychloride followed by sodium borohydride reduction of a nonisolated iminium salt resulting from the cyclization (Scheme 49) <2000BMC2113>. 

A xanthone also results from the cyclization of 2 -nitro-6-hydroxy-2,4-dimethoxyben-zophenone, which is the sole product from a photo-Fries rearrangement of 3,5-dimethoxyphenyl 2-nitrobenzoate (81T209). Ring closure is effected in boiling piperidine and appears to involve elimination of nitrous acid together with ether cleavage. 

The spirolactone (564) results from the cyclization of 4-cyclohexylidenebutanoic acid with sulfuric acid, although in polyphosphoric acid the heterocycle was formed in minor quantities, a hexahydronaphthalenone being the main product (75AJC2669). 

In the case of the (Z)-vinylsilane (Z)-146, the observed syn selectivity results from the cyclization of intermediate 149 in which the TMS group is already axially orientated due to the Z-double bond geometry of the precursor. 

Geometrical (cis-trans) isomers of benz-omorphans result from the cyclization step in their synthesis when the bond to C-6 is formed (Scheme 2). When this is considered a transaddition to the double bond the preponderance of the cis or form is easily explained (10). Resolution of optical isomers from both cis and trans forms has been reported (7,19.20). The norpentazocine precursor in the cis form crystallized first as the (-) base with (+) tartaric acid which would give the twenty-fold more active (-) pentazocine. The (+) base (-) tartarate was recovered from resolution liquors. Quinic acid salts were formed for resolution of the trans isomers. 

The significant gain in selectivity is ascribed to the increased sulfur-induced stabilization of the alkyl radical resulting from the cyclization step. 

Muth et al. studied the hydrogenation and cyclization of 2-nitro-2 -carboxy-biphenyl and its carboxy derivatives (22a-22d) over platinum oxide in ethanol (Scheme 9.13).154 IV-Hydroxyphenanthridone (25) was considered to result from the cyclization at the hydroxylamino compound 23, and phenanthridone (26) to result from the amino compound 24 and not via 25, since 25 did not undergo the reduction to yield phenanthridone under the conditions of the reaction. 

Fully aromatic derivatives result from the cyclizations of compounds of type 29 or of equivalents which can be constructed in a variety of ways, and are often not isolated. 

Thus, both the isocoumarin 38 and the naphthopyrone 39 must result from the cyclization of the common intermediate... 

While unmodified Pd/Al203 was an effective catalyst for the semihydrogenation of the amino alkyne, 9, (Eqn. 16.8), the hydrogenation of the amino diyne, 10, over palladium in absolute ethanol gave the products resulting from the cyclization of the partially saturated triple bonds on the catalyst surface (Eqn. 16.9). ... 

Ammonia gives a broad and diffuse peak. Only traces of pyrazine and dimethylpyrazine are more easily identified. These compounds probably result from the cyclization of the radical fragments formed from the thermal decomposition of the polymer. 

Name the anomers that result from the cyclization of these molecules. ... 

The reaction products of (methoxymethylene)-substituted Meldrum s acid with 2-arylpyrimi-din-6-amines have been used for thermal cyclizations. In Dowtherm at 250 °C, the carboxy group resulting from the cyclization process is eliminated and thus the reaction yields 2-arylpyrido[2,3-[Pg.116]

Benzoxathiazine dioxides 21 result from the cyclization of (2-hydroxyaryl)sulfonamides, which can be carried out using various reagents, e.g. ortho esters or phenylacetyl chloride.64... 

Finally, a radical cyclization/oxidation protocol was utilized as the final step in the Crimmins synthesis of the natural product (-)-talaromycin A.41 Silylation of allylic alcohol 42 produced 43, which cyclized upon treatment with BusSnH and AIBN to provide 44. Oxidation of the crude residue resulting from the cyclization reaction resulted in the formation of (-)-talaromycin A (45). 

This concept turned out to be useful for a variety of transformations, including reductions, epimerizations [14], hydrosilylations of alkenes, hydrostannations of al-kynes [15] and carbon-carbon bond formation [13]. As an example, thiols catalyze the cyclization of acyl radicals [16a] and masked acyl radicals [16b] these reactions have no preparative value in the absence of thiols because the nucleophilic radicals resulting from the cyclization step are not able to abstract efficiently a hydrogen atom from the starting material, whereas thiyl radicals can (Scheme 2). 

Figure 28. Cyclic isoprenoid lipids and their relationships to squalene. (a) Squalene folded so as to demonstrate its relationship to a sterol carbon skeleton (b). In sterols that occur commonly, some of the methyl groups resulting from the cyclization of squalene move from one carbon to another and others are lost. The substituents marked R can be either methyl or H and the substituent marked R can be H, methyl, ethyl, or propyl.

Substituted 4-aminoquinolines, e.g. (131) and (133), result from the cyclization of the substituted o-cyano-aniline (132) the product that is formed depends on the choice of cyclizing reagent (Scheme 54). ... 

Working on the hypothesis that the stereochemical course of squalene biocycliza-tion may be controlled, in part, by stereoelectronic factors rather than by enzymatic conformational influences, a strong asymmetric induction has been achieved in the cyclization of the dienic acetal (97) with SnCl in benzene. The epimeric mixture (98) was oxidized to the octalones (99a and b) which were then separated and their absolute configurations determined. The (stereoselective) yields resulting from the cyclization reaction were 8 and 92 % for (99a) and (99b) respectively. 

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