Cyclic dienes, preparation

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

An interesting addition reaction of sulfur dichloride has been discovered that allows the preparation in good yield of bridged cyclic sulfides from cyclic dienes. Two examples of the reaction have been described (cyclo-octadiene and in the other 1,5-cyclooctadiene. The former sequence is shown in the scheme. The experimental details of the latter sequence are given below. 

Esters and ketones bearing P-nitro groups can be prepared in many ways. For example, the Diels-Alder reaction of methyl P-nitroacrylate is one typical case. Various cyclic dienes are prepared by this route, and the reactions of Eq. 7.121167 andEq. 7.122168 are exemplified. 

Amino alkenols have been prepared by palladium-catalyzed chloroacetoxylation and allylic amination of 1,3-dienes. 1,4-Acetoxychlorination is stereospecific and cyclic dienes give an overall cis- 1,4-addition12. Acetoxychlorination of 6-acetoxy-l,3-cycloheptadiene afforded only one isomer as shown in equation 8. Sequential substitution of the allylic chloro group can occur with either retention or inversion, thereby allowing complete control of the 1,4-relative stereochemistry (equation 9). 

Cyclic (diene)Mo(CO)2Cp (or In) cations have been prepared by trityl cation mediated alkoxide abstraction from cyclic ( 73-allyl)Mo(CO)2Cp (or In) complexes bearing a syn alkoxy in the a position (e.g. 55, Scheme 14)81b 86. Additionally, protonation of (r/3-allyl)Mo(CO)2In (or Cp ) complexes bearing a vinyl group (e.g. 56, Scheme 14) affords the corresponding (diene)Mo+ cations8115 87. 

Enyne metathesis can also be used with highly substituted substrates. Catherine Lievre of the Univcrsite de Picardie reports (J. Org. Chem. 2004,69, 3400) that enynes such as 11, readily prepared from carbohydrate precursors, are cyclized by the second generation Grubbs catalyst 2 to the enantiomerically-pure cyclic dienes, exemplified by 12. 

Reaction of borane with the cyclic diene, cycloocta-1,5-diene, is used to prepare the synthetically useful borane 9-borabicylo[3.3.1]nonane (9-BBN)... 

Such rearrangements have been exploited to homologate cyclic dienes, to prepare bridged annulenes and cyclohepta-l,3,5-trienes (Scheme 3.13).27,28... 

Among cyclic polyenes, cyclic dienes, trienes and tetraenes have been ring-open polymerised via the metathesis reaction. Representative of the cyclodienes most commonly used for polymerisation are 1,5-cyclooctadiene, norbornadiene (bicyclo[2.2.1]hept-2,5-diene) and dicyclopentadiene as mono-, bi- and tricyclic diolefins respectively. Cycloocta-1,5-diene metathesis polymerisation is another approach to the preparation of 1,4-polybutadiene ... 

Cyclic sulfones have been prepared by RCM in what appears to be a general process <02OL427> (Scheme 56). These sulfones readily undergo the Ramberg-Backlund reaction to form cyclic dienes in excellent yields. 

Cyclic diene ether 93 was prepared in high yield starting from lactone 163 through the corresponding enol triflate (Equation 8) C1995TL8263, 1997CL665>. 

Table 7 Transition metal complexes of cyclic dienes/trienes/polyenes/n-donor ligands prepared via the metal vapor-ligand codeposition method...

The dithiocine tetraoxide derived from cyclocondensation of binaphthodithiol with dichloroethylene and oxidation (eq 8) is a chiral version of the bis(phenylsulfonyl)ethylenes. These compounds are useful acetylene equivalents in cycloaddition reactions (see l,2-Bis(phenylsulfonyl)ethylene). Indeed, a chiral acetylene equivalent allows the preparation of optically active hydrocarbons which would be difficult to prepare by classical methods. The dithiocine tetroxide reacts with nonsymmetric dienes to give a single crystalline diastereomeric adduct in most cases. Adducts (1) and (2) were obtained from acyclic and cyclic dienes. 

The transformation of conjugated dienes to allylsilanes can also be carried out with asymmetric induction. The efficiency of the asymmetric synthesis for acyclic dienes is moderate, but for cyclic dienes it remains to be improved. Nevertheless, these reactions provide us with a method for the preparation... 

The general structures of cyclic sulfoximines are reported in CHEC-II(1996) <1996CHEC-II(3)319> (see p. 348). Very few examples of sulfoximines were reported in the decade under review. The preparation of cyclic sulfilimides 165 and 173 from (R)-l-/>-tolyl-lA-isothiazol-3-ones 164 and diazoalkanes and cyclic dienes, respectively, have been reported (see Section 4.05.5.11). Furthermore, studies on the alkylation of a-sulfonimidoyl carbanions and on their use as chiral ligands in copper reagents were reported (see compounds 227 and 228, Section 4.05.6.3.4). 

Additional examples where chloroacetates from acyclic dienes have been used include the synthesis of pentadienylamines [96], dienesulfones [97], c-methylenecyclopentenones [98], marine natural products [99], and the carpenter bee pheromone [82]. Some additional Synthetic applications of the chloroacetoxylation of cyclic dienes are given in Refs. [100-104]. Chloroacetoxylation was also used to prepare a number of starting materials for the intramolecular reactions discussed in this chapter. 

The Diels-Alder reaction is reversible, and many adducts, particularly those formed from cyclic dienes, dissociate into their components at higher temperatures. Indeed, a refro-Diels-Alder reaction is the principal method for preparing cyclopenta-diene prior to its use in cycloaddition reactions. 

Other strained cyclohexa-1,2-dienes incorporated in a polycyclic framework, e.g. 89, can be prepared as intermediates and trapped as their [2 + 2] adducts.Interestingly, [4 -I- 2] adducts, e.g. 91, were obtained with conjugated cyclic dienes. Whether the latter are formed as primary products or arise from secondary transformations of the initially formed [2 -I- 2] adducts is not clear. 

---