Camphor derivatives, chiral auxiliaries

The Lewis acid-promoted [4+ 2]-cycloaddition reaction of the allenic ester 103 having a camphor-derived chiral auxiliary with cydopentadiene provided the adduct with excellent Jt-facial selection, leading to an enantioselective synthesis of (-)-/l-san-talene [92]. 

Sato and coworkers have reported an asymmetric synthesis of Baylis-Hillman-type allylic alcohols 48, 49 via a chiral acetylenic ester titanium alkoxide complex (Scheme 9) [41]. These reactions rely on the use of the novel acetylenic ester titanium alkoxide complex 44 with a camphor-derived chiral auxiliary. Optically active, stereodefined hydroxy acrylates 46, 47 were obtained in high yields and with excellent regio- and diastereoselectivities. The chiral auxiliary was subsequently cleaved off by alcoholysis. 

Scheme 6. A camphor-derived chiral auxiliary for the intramolecular Pauson-Khand reaction...

Pfeiffer and Phillips have demonstrated an interesting RCM reaction between two enone systems, requiring also the Grubbs 11 catalyst (Scheme 16). However, the synthesis starts with a cross metathesis of an enone containing a camphor-derived chiral auxiliary, and a subsequent [4 + 2] with 1,4-dimethylcyclohexadiene. The reaction sequence continues... 

Addition of dicarboethoxycarbene to cycloocta-1,3-diene yields a mixture of cis-and fran -cyclopropane adducts, probably by addition of the singlet carbene on the partially isomerized diene (due to the irradiation). Diastereoselective cyclopropan-ation of a ,/3-unsaturated acetals has been described using a camphor-derived chiral auxiliary. Intramolecular cyclopropenation of a diazo ester, tethered through a naphthalene, to an alkyne was catalysed by rhodium acetate and reported as a efficient method unfortunately, the use of chiral rhodium catalysts gave a less efficient reaction and did not provide high asymmetric induction. ... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

Another application of camphor-based chiral auxiliaries is the use in stereoselective Diels-Alder cycloadditions (Chapter 26), especially for the construction of quaternary carbon centers. One example, shown in Scheme 5.12, uses a camphor-derived lactam 30 as the auxiliary.54... 

Chiral acryloylhydrazide 30, derived from novel camphor-based chiral auxiliary, react with aldehydes in the presence of DABCO to afford p-hydroxy-a-methylene carbonyl derivatives 31 and 32 with practical, high level diaster-eoselectivity (up to 98% de). Moreover, each diastereomer, 31 and 32, can be prepared with high optical purity from the same chiral auxiliary by appropriate choice of reaction conditions (Scheme 2.17). ... 

Optically active bicyclo[2.2,2]octanes can be obtained via diastercoselective MIMIRC reaction of lithium dienolates and a,/ -unsaturated esters of various chiral alcohols. Good yields (70-90%), high endo selectivities (> 95%) and diastereomeric ratios that depend on the auxiliary alcohol are found in these additions. The highest diastereomeric ratio reached was 18 82 using a camphor derived sulfonamide. The diastereomeric ratio could be improved (up to 9 91) by titanium(IV) chloride catalyzed addition of the corresponding silylenolates with the chiral a,/J-unsaturated esters358. 

The addition reactions of alkyllithium-lithium bromide complexes to a-trimethylsilyl vinyl sulfones that have as a chiral auxiliary a y-mono-thioacetal moiety derived from ( + )-camphor are highly diastereoselective. A transition state that involves chelation of the organolithium reagent to the oxygen of the thioacetal moiety has been invoked. The adducts are readily converted via hydrolysis, to chiral a-substituted aldehydes22. 

Imidazole and its derivatives continued to play an important role in asymmetric processes. Optically active pyrroloimidazoles 26 were prepared by the cycloaddition of homochiral imidazolium ylides with activated alkenes <96TL1707>. This reaction was used in the enantioselective preparation of pyrrolidines <96TL1711>. A review of the use of chiral imidazolidines in asymmetric synthesis was published <96PAC531> and the preparation and use of a new camphor-derived imidazolidinone-type auxiliary 27 was reported < 6TL4565> <96TL6931>. 

Camphor-derived sulfonamide can also permit control of enantioselectivity by use of additional Lewis acid. These chiral auxiliaries can be used under conditions in which either cyclic or noncyclic TSs are involved. This frequently allows control of the syn or anti stereoselectivity.143 The boron enolates give syn products, but inclusion of SnCl4 or TiCl4 gave excellent selectivity for anti products and high enantioselectivity for a range of aldehydes.145... 

A new chiral auxiliary based on a camphor-derived 8-lactol has been developed for the stereoselective alkylation of glycine enolate in order to give enantiomerically pure a-amino acid derivatives. As a key step for the synthesis of this useful auxiliary has served the rc-selective hydroformylation of a homoallylic alcohol employing the rhodium(I)/XANTPHOS catalyst (Scheme 11) [56]. 

For acrylates, or type I reagents, applied in asymmetric Diels-Alder reactions, several chiral auxiliaries such as menthol derivatives, camphor derivatives,16,3 and oxazolidinones4 are available. Carbohydrate compounds have also been reported as chiral auxiliaries in a recent publication, although the stereoselectivity was not good.5 Here are examples in which asymmetric Diels-... 

Camphor sultam derivatives have proved to be effective chiral auxiliaries in many different types of asymmetric reactions. As shown in Scheme 5-44, chiral camphor sulfam can be applied in the synthesis of (—)-pulo upone precursor 151 using an intramolecular Diels-Alder reaction. A Wittig reaction of 148 with 147 connects the chiral auxiliary to the substrate, and subsequent intramolecular Diels-Alder reaction via transition state 150 affords product 151. Compound 151 already has the stereochemistry of (—)-pulo upone 153.72... 

Scheme 4.71 Camphor-derived auxiliary for the asymmetric synthesis of chiral allene ether 280.

There are some problems associated with the use of sugar-derived auxiliaries 16 and 18. The nudeophilicity is much lower than for 1, possibly because of the presence of multiple ether functions that can complex lithium ion. It was necessary to include 4 equiv. of LiCl in the addition reactions of 16 and 19 to enamides, otherwise yields were low. A more serious problem associated with 16 is the erosion of the ee of products that was observed when the reactions were scaled up from 0.2 to 4 mmol. Fortunately, a chiral auxiliary that is prepared from camphor does not have these shortcomings [10]. 

Very recently the tandem hydroformylation/acetalization has been used for the synthesis of new synthetically valuable chiral auxiliary derived from camphor. Stereoselective allylation of camphor and subsequent terminal hydroformylation of the resulting homoallylic alcohol affords the 5-lactol auxiliary (camTHP OH) in multigram scale (Scheme 8) [41]. 

The formation of spirocyclopropanes from the reaction of diazodiphenylmethane and ( )-8-phenylmenthyl esters of acrylic acid and methyl fumarate occurred with a modest level of diastereofacial selectivity (136). In contrast, diastereoselectivities of 90 10 were achieved in the cycloadditions of diazo(trimethylsilyl)methane with acrylamides 65 derived from camphor sultam as the chiral auxiliary (137) (Scheme 8.16). Interestingly, the initial cycloadducts 66 afforded the nonconjugated A -pyrazolines 67 on protodesilylation the latter were converted into optically active azaproline derivatives 68. In a related manner, acrylamide 69 was converted into A -pyrazolines 70a,b (138). The major diastereoisomer 70a was used to synthesize indolizidine 71. The key step in this synthesis involves the hydrogenolytic cleavage of the pyrazoline ring. 

W. Oppolzer, Camphor Derivatives as Chiral Auxiliaries, in Asymmetric Synthesis, Tetrahedron 43, 1969 (1987). 

It was clear that 1 would be derived from a Diels-Alder adduct. There has been a great deal of work in recent years around the development of enantioselective catalysts for the Diels-Alder reaction, but the catalysts that have been developed to date only work with activated dienophile-diene combinations. For less reactive dienes, it is still necessary to use chiral auxiliary control. One of the more effective of those was the known camphor-derived tertiary alcohol, so that was used in this project. Diels-Alder cycloaddition of the diene 4 with the enantiomerically-pure enone 5 led to the adduct 6 with high diastereocontrol. Oxidative cleavage led to the acid 7, which was carried on to the bis-enone I. 

The amino alcohol-catalyzed enantioselective addition of dialkylzincs to aldehydes, detailed in Chapter 5 (27), is accomplished with polymer catalysts containing DAIB, a camphor-derived auxiliary, and other chiral amino alcohols (28). Reactions that involve matrix isolation of the catalyst not only result in operational simplicity but also greatly facilitate understanding of the reaction mechanism. In solution, the catalytic chiral alkylzinc alkoxide derived from a dialkylzinc and DAIB exists primarily as dimer (27) however, when immobilized, its monomeric structure can be maintained. 

A 1,3-oxathiane chiral auxiliary derived from camphor has also been reported [58]. 

In an isolated case, a tertiary alcohol was used as chiral auxiliary the crotonate (19), prepared from the (+)-camphor-derived alcohol (18), undergoes highly diastereoselective 1,4-addition of a cuprate reagent (Scheme 6).24... 

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

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