Carbocycles/heterocycles, five-membere

Dicarborative telomerization has also been successfully applied to intramolecular cyclization reactions. Various tetraenes with enamines, indole, nitroalkanes or carbonyl-stabilized carba-nions give good yields of carbocyclic or heterocyclic five-membered rings, with predominantly the trans arrangement of the two different unsaturated side chains14,15. 

Cycloadditions of320 have been studied thoroughly. Depending on the reaction partners different carbocyclic and heterocyclic five-membered ring systems were obtained. 

Dipolar cycloaddUions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [7, 2] The versatility of this class of pericychc reactions in the synthesis of five-membered-ring heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-ring carbocyclic systems (equation 1)... 

Asymmetric Pauson-Khand reaction in syntheses of heterocycles fused with five-member carbocyclic fragment 980PP121. 

In many ways, the electron-rich five-membered aromatic heterocycles behave very much like carbocyclic aromatic compounds when it comes to lithiation. Lithiation a to O or S of furan and thiophene is straightforward (Scheme 130) . The usual selection of orf/io-directing groups allows lithiation at other positions and some examples... 

This chapter reviews the systems where account is taken of the size of the central carbocyclic ring, the relative orientation of the fused five-membered heterocyclic rings, the types of heteroatoms, their number, and, in rings with more than one heteroatom, their relative situations. 

This section follows the content of Section 9.02.8 in CHEC-II(1996) without repetition of data presented in the latter but refers to several papers omitted in CHEC-II(1996) along with new publications. Below are presented various synthetic methods, which are classified by the nature and size of the starting ring. Peculiar differences with CHEC-11(1996) should be marked (1) the absence of paragraphs devoted to formation of oxepine derivatives from four- and five-membered carbocycles because new syntheses of these kinds were not found in the literature (2) Sections 13.02.8.2.2, 13.02.8.2.5, and 13.02.8.2.6, which are new, appear concerning with formation of oxepine derivatives from four-, (other) seven-, and eight-membered heterocycles, respectively. 

We will take a semiempirical approach using numerous molecules, models, assumptions, and estimates rather than doing new calorimetric experiments and/or quantum chemical calculations. Indeed, we will also test what is probably the simplest assumption - that (4n + 2) n electrons found within a conjugated ring species is expected to result in enhanced stability and that this compound is called aromatic. We will consider the dihydroindene (indane) skeleton composed of a benzene ring fused to a nonaromatic five-membered ring that lacks additional double bonds, and will use this carbocyclic hydrocarbon with X = Y = Z = CH2 as a paradigm for many heterocyclic derivatives for which the possible aromaticity is of relevance to the current chapter. Similarly we use indene with -X-Y- = -CH=CH-, Z = CH2 for a variety of unsaturated heterocycles of interest here. 

As an extension, carbocycles or heterocycles 35 are synthesized by intramolecular hydrocarbonation of alkyl- or alkoxyallenes 34 bearing active methyne groups at the terminus of the carbon chain, respectively (Scheme 7). With alkylallenes five-membered ring formation proceeds smoothly comparing to six-membered ring formation [6g] this problem is not encountered with alkoxyallenes [6h]. 

Since the classic papers by Ingold and his co-workers,110, 111 nitration has for a long time been considered as the standard electrophilic substitution. Many orientation and relative rate data on the nitration of both carbocyclic and heterocyclic substrates have been accumulated and the results have been generalized as valid for all electrophilic substitutions. As a matter of fact, this popularity is partially undeserved nitration is a complicated reaction, which can occur by a multiplicity of parallel mechanisms.112 In particular, in the case of the very reactive substrates that five-membered heterocycles are, two complications may make meaningless both kinetic measurements and competitive experiments.113 (i) Due to the great reactivity of both partners the encounter limiting rate may be achieved in this case, of course, all the substrates react at the same rate and the effect of structure on the reactivity cannot be studied. (ii) Nitrous acid, always present in traces, may exert an anticatalytic effect in some cases and a markedly catalytic effect in others with a very reactive substrate, nitration may proceed essentially via nitrosa-tion, followed by oxidation. For these reasons, the nitration data must be handled with much caution. 

Synthesis of Five-Membered Carbocyclic and Heterocyclic Ring Systems... 

Dipolar cycloaddition of nitrile oxides to unsaturated five-membered carbocyclic and heterocyclic systems gives fused 1,2-oxazolines. Steric effects are apparently of far greater importance than electronic ones, especially with regard to the orientation of the dipolarophile towards the nitrile oxide in the cycloaddition reaction. Originally, only one... 

The intramolecular addition of organolithiums to non-activated carbon-carbon double or triple bonds has now become an efficient way of constructing carbocyclic and heterocyclic systems. Although mainly confined to the formation of five-membered rings, the high regio- and stereoselectivity of these reactions and the possible functionalization of the... 

---