Acyclic starting materials

The direct preparation of pyridinium salts (35) from acyclic starting materials 33 and 34 was reported by Adamczyk and co-workers <00TA2289>. Pyridinium salts have also been used to form other heterocyclic ring systems such as imidazolo[l,2-a]pyridines as reported by Katritzky <00JOC9201>. A final report worthy of comment is the preparation of chiral 2,6-disubstituted tetrahydropyridines via chiral pyridinium salts . 

In the field of dihydropyridines, chiral difluoro ester 237a was obtained by RCM (10OL3484) whereas simpler 237b-type esters could even be composed from acyclic starting materials in an efficient one-pot protocol (four-component-reaction / y-alkylation / RCM / elimination sequence) (10JCO713). 

Cycloaddition to endocyclic unsaturation has been used by many researchers for the preparation of isoxazoUdinyl adducts with y-lactams derived from pyrogluta-minol and is discussed later in this chapter as a synthesis of unusual amino acids (Scheme 1.20, Section 1.6) (79,80). A related a,p-unsaturated lactam has been prepared by a nitrone cycloaddition route in the total synthesis of the fungal metabolite leptosphaerin (81). A report of lactam synthesis from acyclic starting materials is given in the work of Chiacchio et al. (82) who prepared isoxazolidine (47) via an intramolecular nitrone cycloaddition reaction (Scheme 1.11). 

We can use this model in retrosynthetic analysis quite successfully. Suppose one were asked to produce cyclohexanone C from acyclic starting materials. 

Direct, reliable methods for synthesis of mesoionic 1,2,3,4-oxatriazoles 4-6 are well established. All rely on the transformation of acyclic starting materials. Thus, 3-alkyl-anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides 4 (R1 = alkyl) are readily obtained by nitrosation of semicarbazides followed by cyclization (Section 6.08.9). The 3-aryl-substituted analogues 4 (R1 = aryl) are prepared either by cyclization of arylazonitromethanes or by nitrosation and cyclization of arylhydrazonomethanesulfonate salts <1984CHEC(4)579, 1996CHEC-II(4)679>. 

The typical preparation of the cyclic boroles from acyclic starting materials usually involves acid base chemistry or transacetalization protocols. 1,3,2-Diazaborolidines, such as 311 and 313, are readily available via the reaction of the corresponding amines 310 and 314 with boron tribromide (BBr3) and boron halide 312 (Scheme 51) <2006JCD3777>. 

The piperidine ring system is found in numerous biologically active alkaloids and a number of reviews have been published on synthetic methods to form piperidines, generally from acyclic starting materials <2000S1781,... 

The construction of the macrobicyclic system is shown in Scheme 5.1. The acyclic starting materials are the diacid chloride 2 and the diamine 3.4 An alternative synthesis of the intermediate monocycle is given in Chapter 2 (Protocol 5). 

This process enabled the synthesis of cyclopeptide [183] and macrolide [184] model compounds with ring sizes up to 37. The acyclic starting materials were conveniently cyclized, with yields between 25 and 50 %. One major problem connected with this thioether method is the incorporation of the electron-donating group into the macrocyclic ring systems. A practical way of overcoming this dis-... 

The generally accepted mechanism of the Ramberg-Backlund reaction (Scheme 39) involves the deprotonation of the a-chlorosulfone by the base to give the sulfonyl carbanion (92). The latter undergoes an intramolecular nucleophilic attack on the a-carbon atom with elimination of the chloride anion and formation of the episulfone (93), which is unstable and extrudes sulfur dioxide to yield the alkene (Scheme 39).6 The Ramberg-Backlund reaction, unlike the Julia reaction, yields mainly the (Z)-alkene from acyclic starting materials. 

This section details syntheses of the pyrrolo[i,2-a]pyrimidine ring system in which only acyclic starting materials are used. In most cases, however, a substituted pyrrole or pyrimidine monocyclic structure is probably an intermediate in this process. These react further under the conditions employed to give the observed bicyclic product. Wollweber et al.31 have provided experimental evidence for just such an intermediate (see below). 

The loss of rotational freedom when the cyclic transition state is formed from the acyclic starting material accounts for the observed negative entropies and volumes of activation. It has been established that the transition state of the closely related Cope rearrangement (see below) resembles the chair conformation of cyclohexane. An analogous transition state structure for the Claisen rearrangement would account for the fact that isomerization of vinyl a-methylallyl ether yields 97% trans 4-hexQm and 3% cw-4-hexenal, since there should be a preference for an equatorial orientation of the methyl group in a cyclohexane chair-like transition state . 

Write the synthesis (not the mechanism) for 2-methyl-3-ethylpyrrole from acyclic starting materials. 

Write the stmctures for the two starting materials to prepare 2-methylfuran (from acyclic starting materials). 

Mukai accomplished the stereosective synthesis of three lycopodium alkaloids (-)-magellanine, (+)-magellaninone, and (+)-paniculatine from a common intermediate. Their strategy employed the use of two intramolecular PKRs to build a tetracyclic intermediate from acyclic starting material diethyl L-tartrate. 

Q-methylvitexin. A similar acyclic starting material (65) was converted into the nitropropyl glycoside (66) as indicated in Scheme 28/" and related methodology was applied to obtain the... 

Propose a synthesis of the following compounds from acyclic starting materials. (Difunctional Relationships-15)... 

Pyridines can be made by condensation reactions of acyclic starting materials such as carbonyl compounds with ammonia. The most general of these methods is the Hantzsch pyridine synthesis. In this reaction, two molecules of a j8-dicarbonyl compound, an aldehyde, and ammonia combine in several steps (Worked Example 25-28) to give a substituted dihydropyridine, which is readily oxidized by nitric acid to the aromatic system. When the j8-dicarbonyl compound is a 3-ketoester, the resulting product is a 3,5-... 

Using 1,5-diynes as precursors to o-xylylenes, whidi then are trapped by Mej SiC2 SiMe, or trapped intramolecularly by using diynes substituted with side chains bearing potential dienophiles, polycyclic systems are generated in a one-pot reaction from acyclic starting materials (Funk and Vollhardt, 1976). 

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