Bomane derivatives

Nickel(0)-catalyzed codimerization of methylenecyclopropanes with electron-deficient olefines are highly regiospedfic, but show a rather poor stereoselectivity. Thus the asymmetric nickel(0)-catalyzed codimerization of methylenecyclopropanes with the chiral bomane derivatives of acrylic acid leads to the optically active 3-methylenecyclopen-... 

Rule 4. To number positions in unsaturated structures named as bomane derivatives, assign the double bond as low a number as is consistent with the fixed numbering of the saturated carbon skeleton. Thus ... 

According to recent investigations, the commercially available oils derive from Artemisia herba-alba Asso (Morocco). Their qualitative composition is similar to the oil of Artemisia vulgaris, greater deviations have been observed quantitatively [165]. Nano et al. [166] demonstrated varying compositions in essential oils of various plant samples from Piedmont. Vulgarol, a bomane derivative, was detected as a new constituent in some of the oils. Spathulenol and a-cadinol, two sesquiterpene alcohols, as well as davanone were found by fork and co-workers [167, 168, 169]. 

There are regularly publications concerning the transformation of bomane derivatives [particularly camphor (40) oxime] to campholenyl derivatives, but there is no particular interest attached to them as syntheses of natural products. Nevertheless, we mention work on the Beckmann-type rearrangements of camphor oxime,and a route to campholenic alcohol (352) from the hydroxycamphene 353. "Y-Campholenol (354) has been identified in Tanacetum... 

Other oxazolidinones have been used as chiral auxiliaries in asymmetric aldol reactions. Bomane derivatives 1.121 (X = O or S) and 1.122 are readily transformed into V-acyl derivatives. The reactions of their boron or titanium enolates with aldehydes give the same selectivities as Evans s reagents [426, 428, 429, 431, 436], iV-Acylimidazolidinones 1.131 and 1.132 [449, 1270] lead to similar results, but the selectivities observed are somewhat lower. 

A rather rare case is the use of chiral nitrile oxide, derived from N-glyoxyloyl-(2i )-bomane-10,2-sultam (211). Several nitrile oxides of the latter type, bearing... 

It has been pointed out that there is a torsional-angle (t) dependence of 3-SCS (cf. 40) (104). This angular dependence can be adequately described by multiplying the second term in eq. [15] by cos t (0° t 90°) for the appropriate fragment (104). By that treatment the 3-SCS values of methyl, hydroxy, chlorine, and nitro substituents in bomane (93,104,146,152-154) and twistane derivatives (104,155), which cannot be predicted by the simple eq. [15], become understandable. As expected, such an angular dependence does not exist if t is larger (90° < t < 180°) (156). 

Optically pure 7-oxanorbom-5-en-2-yl derivatives ("naked sugars") are readily available. Substitution of their centers C(3), C(5) and C(6) can be done with high stereo- and regioselectivity in a predictable fashion. The polysubstituted 7-oxanor-boman-2-ones so-obtained can be transformed into D- or L-carbohydrate derivatives, C-nucleoside precursors or polysubstituted cyclohex-2-enones and cyclohexenols (Scheme 12). Stereoselective rearrangements of the 7-oxanorbom-2-yl systems into polyhydroxylated cyclopentyl derivatives are also possible. 

Langlois and co-workers (179) found the same exo stereochemical preference through double asymmetric induction of a related ene-lactone (/ )-145 with their well-explored and efficient camphor-derived oxazoline nitrone (1S)-146 (Scheme 1.32). They found the cycloaddition components form a matched pair and allowed kinetic resolution of the racemic lactone in up to 70% enantiomeric excess (ee). They suggest the selectivity for exo adduct 147 arises through destabilization of the endo transition state by a steric clash between dipolarophile ring hydrogens and the bomane moiety. 

The scope of this problem can be indicated by a brief consideration of the stereochemical nomenclature of hydroxy derivatives of the bicyclic terpenes. For the parent bicyclic hydrocarbon compound known in the literature at present as cam-phane, the name "bomane is recommended in this report. (Reasons for choosing "bornane are discussed later under "The Bornane Hydrocarbons .) A mono-hydroxy derivative of bornane, on the basis of IUPAC rules (41), would be named as a bornanol. Two such derivatives have the common terpene names, bomeol and isobomeol these names, on the basis of recommendations in this report would both become 2-bornanol. Obviously, this is inadequate and additional designations are necessary to distinguish between die two stereoisomers. 

The bicyclic terpene hydrocarbons classified in this report as the bomane group (bicycloQ2.2.1 heptane type), but currently called the camphane group in terpene literature, are more numerous and have many more important derivatives (especially the camphors) than the other three fundamental bicyclic types — the thujane, carane, and pinane groups — already discussed. As would be expected, a greater variety in the arrangement of substituents and double bonds and in the number of substituents is found. 

These and other considerations led directly to the recommended name bomane group for the group of bicyclic terpene hydrocarbons currently called the camphane group. Specific recommendations on continued usage of the common terpene names of bornane-type bicyclics are being withheld until a more thorough study has been made of the nomenclature of their derivatives. 

Bomane Group (BicycloQ2.2.IQ heptane Type). Name the bomane-type bicyclic terpene hydrocarbons (bicycleheptane type) as norbomanes, norbomenes, and norbomadienes and base these names on systematic bicyclo numbering and rules (Formula 73, Chart 9). For those derivatives which have a carbon skeleton identical to bomane, use the bomane, bomene, and bornadiene names, and base these names on the recommended fixed numbering of the bomane carbon skeleton (Formula 75, Chart 8). See Chart 14. 

Chiral cyclopentanones.2 The regioselective cyclization of a-diazo-p-keto esters to cyclopentanones (11, 459) is also enantioselective with substrates derived from chiral alcohols. Preliminary studies show that steric factors affect the diastereoselectivity the highest diastereoselectivity is obtained with esters of the alcohol 1, which is available from camphor,3 and in which both the bomane and the naphthalene rings can exert steric effects on the diastereoselectivity. 

Introduction. One of several auxiliaries that exploit the asymmetry of naturally occurring (-b)-camphor, the 3-(N-(3,5-dimethylphenyl)benzenesulfonamido)bomeol auxiliary has proven significant utility in the jt-facial differentiation of ester enolates and enoate derivatives. The endo orientation of the C(2) and C(3) substituents places the reactive functionality within the concave pocket created by the bomane skeleton as well as the shielding ability of the M-arylbenzenesulfonamide. 

Bomane-10,2-sultam (362) and its antipode, accessible from inexpensive (+)- and (-)-camphorsulfonic acid in two simple operations, were introduced in 1984 and rank today among the most practical auxiliaries (Scheme 88). Both chirophore enantiomers are commercially available in kg quantities. Almost all of their A -acyl derivatives are stable and can be (1) readily purified by crystallization, (2) directly analyzed by H NMR and/or GC to determine their stereochemical purity, and (3) cleaved e.g. with UAIH4, LiOH, LiOOH, MeOMgl, etc.) under mild conditions without loss of the induced chirality and with excellent recovery of the auxiliary. 

Enantiopure P-alkoxyalcohols have also been used as chiral auxiliaries. Among the most useful auxiliaries in this class are the acyclic methylether 1.11 [148] or silylether 1.12 [149, 150], and the cyclic monobenzyl- or neopentylethers 1.13 (R = PhCH2 or tert-BuCH2) derived from exo-bomane-2,3-diols [147], From inexpensive, commercially available ephedrine, (li , 2S)-Af-methylephedrine 1.14 is easily obtained, its (15,2R)-enantiomer is easily available too. The esters of these alcohols [151-154] have frequently been used. Among the cyclic alcohols, sulfonamide derivatives 1.8 and 1.9 (R = NHS02Ar or N(Ar)S02Ph) are especially useful [147,155],... 

Oppolzer and coworkers [147, 454] have developed a class of reagents based on the enantiomeric bomane-2,10-sultam skeleton 1.133. These chiral auxiliaries are easily prepared from the enantiomeric 10-camphosulfonic adds [455]. Saturated or a,P-unsaturated TV-acylsultams 1.134, occasionally prepared from Af-silyl precursors [396], have been used very frequently. Asymmetric alkylations, animations and aldol reactions of enolates or enoxysilane derivatives of 1.134 (R = R CH2) [147, 404, 407, 456-460] are highly selective. The a,(3-unsaturated TV-acylsultams 1.134 (R = R R"C=CH) suffer highly stereoselective organocuprate 1,4-additions [147, 173], cyclopropanations [461], [4+2] and [3+2] cydoadditions [73,276,454,462], OSO4 promoted dihydroxylations [454,463] and radical addi-... 

Both 2,3-e iio,eM(io-dimethylnorboman-7-one 23 and 2,3-enc o,e iio-diethylnor boman-7-one 24 are anti-selective. However, the anti-selectivity of the diethyl derivative is significantly superior to that of the dimethyl derivative. For instance, on reduction with LiAlH4, the anti syn selectivity is 79 21 for 24 and only 55 45 for 23. If cjvicinai —> a interaction, as advocated by Cieplak, is indeed the control element, both molecules will be predicted to exhibit syn selectivity because (a) crc H is more electron-donating than cjc c and (b) the anti side has two endo Cc-h bonds in lieu of the two endo cjc c bonds on the syn side. Consequently, the Cci ce and CJC4-C5 bonds, both on the anti side, must be more electron-rich than the tfci-C2... 

An interesting asymmetric aldol reaction utilizing enantiomerically homogeneous bomane sultam derived boron enolates has recently been reported by Oppolzer et al. The reaction of aldehydes with boron enolates (57), generated from acyl sultams (58) under standard enolization conditions (Pr 2NEt/Bu2BOTf/0 C), provides syn aldol products (59) with extremely high ratios of (59) to (60) as shown in Scheme 29. Results from the aldol reactions with representative aldehydes are summarized in... 

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