Stereochemical Indications

APCI-MS has been used extensively for the structural analysis of diterpene alkaloids from various Aconitum species for both positional and stereoiosomeric determination [ 57,58]. By controlling the drift voltage between the first and second electrodes in 

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Fig. 7.3.10. Biogenesis of neolignans belonging to the bicyclo[3.2.1]octanoid (17, 23), futoenone (18,24), porosin (19), burchellin (25), and megaphone (20, 21) groups. Stereochemical indications are omitted in order to maintain the information as generally valid as possible...

Figure 2.22 Left Two substituents attached to a saturated ring are designated with the relative stereochemical indicator cis if both are on the same face of the ring and trans if on opposite faces. Right When there are two substituents attached to the same ring carbon, then a substituent located elsewhere on the ring is c/s if it is on the same face as the substituent of higher priority.

Several examples will sufEce to demonstrate the naming of these compounds. Codeine phosphate (Figure 6.65) is a mild analgesic and a cough suppressant. Its systematic name is morphinan-6-ol, 7,8-didehydro-4,5-ep-oxy-3-methoxy-l 7-methyl-, (5a, 6a)-, phosphate (1 1) salt. In the name, we see the parent morphinan listed as well as several substituents. The stereochemical indicators are listed after the name and again we see a- and P-used to indicate the orientation of substituents. Also note that codeine is a 1 1 salt with phosphoric acid. Once the basic morphinan structure is drawn, several familiar substituents such as an alcohol, and a methoxy and a methyl... 

Given the following partial structure add a substituent X to C 1 so that It satisfies the indicated stereochemical requirement You may find it help ful to build a molecular model for reference... 

The S in S adenosylmethio nine indicates that the adenosyl group is bonded to sulfur It does not stand for the Cahn-Ingold-Prelog stereochemical descriptor... 

CIS (Section 3 11) Stereochemical prefix indicating that two substituents are on the same side of a nng or double bond (Contrast with the prefix trans )... 

Specifications and standards for various vitamin E forms and preparations for use in pharmaceutical appHcations ate given in the United States Pharmacopeia (52). AH products should contain not less than 96.0% or more than 102.0% of the appropriate form. The products must be labeled to indicate both the chemical and stereochemical forms contained in the product. 

The structure of a natural product is shown without any specification of stereochem-istiy. It is a pure substance which gives no indication of being a mixture of stereoisomers and has zero optical rotation. It is not a racemic mixture because it does not yield separate peaks on a chiral HPLC column. When the material is completely hydrolyzed, it gives a racemic sample of the product shown. Deduce the complete stereochemical structure of the natural product fiom this information. 

The rate-determining expulsion of bromide ion through a bridged intermediate requires an anti orientation of the two bromides. The nucleophilic attack of iodide at one bromide enhances its nucleophilicity and permits formation of the bridged ion. The stereochemical preference in noncyclic systems is also anti, as indicated by the fact that /neso-stilbene... 

When this prior stereoi merization is accounted for, the rearrangonent is found to have resulted fixtm a mixture of all possible suprafacial, antarafacial, inversion, and retention combinations in reughly equal amounts, indicating that no stereochemical pathway is strongly preferred. Substituted systems, however, show higher stereoselectivity. Theoretical modeling of the reaction finds no intermediate, but tire titumtinn state is diradical in character. ... 

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Newman projection (Section 3.6) A means of indicating stereochemical relationships between substituent groups on neighboring carbons. The carbon-carbon bond is viewed end-on, and the carbons are indicated by a circle. Bonds radiating from the center of Ihe circle are attached to the front carbon, and bonds radiating from the edge of the circle are attached to the rear carbon. 

A sequence of straightforward functional group interconversions leads from 17 back to compound 20 via 18 and 19. In the synthetic direction, a base-induced intramolecular Michael addition reaction could create a new six-membered ring and two stereogenic centers. The transformation of intermediate 20 to 19 would likely be stereoselective substrate structural features inherent in 20 should control the stereochemical course of the intramolecular Michael addition reaction. Retrosynthetic disassembly of 20 by cleavage of the indicated bond provides precursors 21 and 22. In the forward sense, acylation of the nitrogen atom in 22 with the acid chloride 21 could afford amide 20. 

The conversion of a thiolactone to a cyclic ether can also be used as a key step in the synthesis of functionalized, stereochemically complex oxacycles (see 64—>66, Scheme 13). Nucleophilic addition of the indicated higher order cuprate reagent to the C-S double bond in thiolactone 64 furnishes a tetrahedral thiolate ion which undergoes smooth conversion to didehydrooxepane 65 upon treatment with 1,4-diiodobutane and the non-nucleophilic base 1,2,2,6,6-pentamethylpiperidine (pempidine).27 Regio- and diastereoselective hydroboration of 65 then gives alcohol 66 in 89 % yield after oxidative workup. Versatile vinylstannanes can also be accessed from thiolactones.28 For example, treatment of bis(thiolactone) 67 with... 

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