Stereochemistry, assignment

Amorphanes 38-40 were also isolated from Acanthella cannabina. These are trans-fused and functionalized at C-l rather than C-10 which was the case with the Halichondria metabolites. The carbocyclic skeleton was demonstrated when 39 was dehydrogenated to yield cadalene (37). Relative stereochemistry assignments were secured by extensive 13C- and XH NMR experiments [39], in which 13C data were particularly useful, as illustrated in the case of isothiocyanate 41, one of several isothiocyanates obtained from Acanthella pulcherrima [20]. The ds-fused ring was corroborated by comparing the corresponding chemical shifts with those of enf-epicubenol (42) [20], which has the same relative stereochemistry. 

The absolute configurations of sclareol (2) and manool (5), Fig. (7), at C-13 have been determined [133]. As Hanson reported [134], the absolute stereochemistry assigned to some labdanes should be reexamined due to the enantiomers of labdanes. The biosynthetic pathways of sclareol and manool start from a geranyllinalool - type skeleton which cyclizes in a similar fashion as that described for cativic acid [135] and, via the intermediate 6, Fig. (7), sclareol is formed by hydration of 6, or manool by the loss of a proton [112,136]. 

To demonstrate the feasibility of Diels-Alder cycloaddition using y-pyrones as dienophiles,47 50 the reaction of 3-cyanochromone 31 with diene 32 in toluene proceeded at 200 °C in a sealed tube for 72 h to give the desired cycloadduct 33 in 80% yield [Scheme 6] without observing any inverse electron demand [4 + 2] cycloadducts.51 However, the endo exo ratio was only 1.3 1 as determined by ]H NMR with the stereochemistry assigned using nOe experiments. 

Ribe S, Kondru RK, Beratan DN, Wipf P (2000) Optical Rotation Computation, Total Synthesis, and Stereochemistry Assignment of the Marine Natural Product Pitiamide A. J Am Chem Soc 122 4608... 

The title compounds, isolated from Berberis buxifolia Lam. (Berberidaceae), are the most recently reported of the eight known bisbenzylisoquinoline /V-oxide alkaloids. They are noteworthy in being the first N-oxides of this group to have their complete stereochemistry assigned by NMR NOEDS studies (see Section VI,A,2,b). Unfortunately, the first report (59) was based on an incorrect assignment due to overlap of the NMe and aliphatic signals of calafatine (88, Section... 

Pu G.-Q., Yamamoto M., Takeuchi Y., Yamazawa H. and Ando T. (1999) Resolution of epoxydienes by reversed-phase chiral HPLC and its application to stereochemistry assignment of mulberry looper sex pheromone. J. Chem. Ecol. 25, 1151-1162. 

NaBH4 reduction of compound (42) opens both heterocyclic rings giving the 1,2,3,4-tetra-substituted cyclobutane (43), whose stereochemistry assigns the configuration of the precursor (42) <71JA3493>. 

The first enantioselective total synthesis of ( )-7,8-epoxycembrene C (33) was achieved via a general approach by employing an intramolecular McMurry coupling and Sharpless asymmetric epoxidation as key steps from readily available starting material. The syntheses presented here verified the absolute stereochemistry assignment of the epoxy configuration of 33 as assumed (1R,8R) (Scheme 6-20). °... 

Oxadiazines 7 are produced as a single diastereomer by the reaction of aryl(benzoyl)diazene and aroyl(ethoxycarbonyl)diazene with enamines of 4-rcrr-butylcyclohexanone. by heating the initially formed 2-substituted enamines 620 21. The equatorial disposition of the C-2 and C-4 substituents was shown by H-NMR spectroscopy, whereas the c/s ring junction and the axial disposition of the C-2 proton is assumed by analogy with the stereochemistry assigned to analogous oxadiazines. Isolation of the oxadiazines from the reaction of morpholinyl enamines and aroyl(ethoxycarbonyl)diazene is facilitated by their precipitation in the reaction medium21. 

Often there is occasion to add a third inactive isomer to these two optical antipodes devoid of rotary power in all its states, this inactive isomer furnishes holoedral crystals stereochemistry assigns it a formula which is reproduced, identical with itself, by reflection in a mirror. 

An X-ray analysis of rastevione, the main constituent of the roots of Stevia serrata and S. rhombifolia, has revealed it to have structure (284).144 This definitive assignment calls into question the relative stereochemistry assigned to more than fifteen other longipinene and longipinane derivatives isolated from various plant sources (see Vol. 7, p. 75, Vol. 9, p. 110, Vol. 10, p. 34). The tiglate (285) has been isolated from Eupatoriadelphus purpureus.lib... 

Both enantiomers of E-3710 (318), a novel analgesic, were prepared and their absolute stereochemistries assigned their pharmacological properties were compared with those of the racemic and no significant difference was observed <93BMC269). 

Isoaltholactone (119) has been obtained from G. malayanus, G. montanus and G. tapis and its relative stereochemistry assigned from NMR data and X-ray analysis. A synthesis from L-arabinose provided the absolute stereochemistry of (119). The biosynthetic pathway in Scheme 2 was proposed to account for the formation of altholactone (114) and isoaltholactone (119) from the same postulated precursor, 5-hydroxygoniothalamin (105) (124). 

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