Mark A. Minton

Asymmetric induction in the ene reaction of glyoxylate esters of 8-phenylmenthol

Abstract We recently communicated (J.K. Whitesell, A. Bhattacharya, D.A. Aguilar and K. Henke, J. Chem. Soc. Chem. Commun. 989 (1982)) a highly efficient and effective method for the control of absolute stereochemistry through asymmetric induction in the ene reaction the chiral glyoxylate 1 with alkenes. We now have accumulated sufficient information on this process in terms of both its mechanistic details as well as its scope and applicability to a variety of situations that warrants a more complete presentation of these reactions.

α-Helical Polypeptide Films Grown From Sulfide or Thiol Linkers on Gold Surfaces

We prepared two new linkers, S-functionalized adamantane derivatives 2 and 3, which bind as monolayers on polycrystalline gold. From these surface anchors, both L- and D-isomers of alanine can be grown as thin films of α-helical polypeptides directed from the gold surface by using the appropriate N-carboxyalanine anhydride. FT-IR Studies show that these layers are roughly 1000-A thick and that, under the same growth conditions, the L-polypeptide layers grow at a rate ca. 30% greater than that of the non-natural D-amino acid. X-Ray photoelectron spectroscopy studies show that, upon equilibration, all three S-atoms of the sulfide moieties of 2 are bound to the gold surface, and that, on average, three of the four thiols of 3 are chemoadsorbed. The essential role of H2O on the surface of these films as a necessary component in these gas-phase polymerization reactions is demonstrated.

Bicyclooctanes: cis-bicyclo[3.3.0]octa-2,6 and 2,7-diene-2-carboxylic acids

Abstract Two independent routes to each of the title acids, useful in the synthesis of natural products, are presented. The sequences commence with readily available materials which are amenable to the preparation of multigram quantities. Complete 13 C spectral data is supplied for intermediates and products.

Bicyclo[4.3.0]nonanes

Substantial data are available for both the cis- and the trans-fused systems. Exo and endo are used for cis-bicyclo[4.3.0]nonanes, as well as for those having a double bond emanating from a bridgehead (though these are listed with the trans-fused compounds). For the trans-fused comounds, the orientation of the hydrogen on C-1 is defined as alpha (A), while beta (B) refers to the orientation of the C-6 hydrogen. Groups are then defined based on the side of the molecule that they are on.

Bicyclo[2.2.1]heptanes

Nomenclature for this system is straightforward, with exo and endo referring to the orientation of substituents on carbons 2, 3, 5, and 6 relative to C-7, and syn and anti used for C-7 substituents. For 7-monosubstituted compounds, the numbering is defined so that the substituent will be syn. Relatively unusual dihedral angles are found about most of the bonds and thus caution should be exercised in applying Δδ effects observed here to other systems.

Bicyclo[4.4.0]decanes

Both the cis- and trans-fused compounds represent valuable models for other, cyclohexane-based systems. Stereochemistry in the cis compounds is specified simply as exo and endo, while the orientations of groups in the trans series and those with a bridgehead double bond are specified based on their orientation as axial (A) or equatorial (E) substituents. Note that this system is distinct from that used with the trans-fused bicyclo[4.3.0]nonanes but it was felt that this inconsistency would be more than repaid here by the ease with which the spatial orientation of the groups could be visualized. Corresponding use of axial and equatorial for the [4.3.0]nonanes would have required new definitions of orientations of substituents on the five-membered ring.

Bicyclo[3.1.1]heptanes

The terms exo and endo can be readily used for the cis-fused bicyclo[3.2.0]heptanes (no data for trans-fused compounds was found).

Multiple Substituent Shift Additivity

The relatively simple table of shift effects provided in Chapter 1 can be used to make approximate predictions of chemical shifts which in turn are useful in making first inspection assignments between carbons and absorptions. It is of course implicit in such a treatment that the magnitude of the shift-of-shift effect for a substituent be relatively independent of the presence of other groups. Thus, the shift of a given carbon might be considered to be the sum of the independent effects of all of the neighboring groups. This does indeed hold true in those situations where substituents do not directly interact with each other and where the conformation of the system is relatively constant. In theory it should be possible to arrive at predicted spectra for all possible isomers of an unknown compound and comparison of these with the observed values would narrow the possible candidates to one or at most two choices. However, it should be clear from the discussion in Chapter 2 that the magnitude of the shift effect of a substituent varies with the carbon framework to which it is attached, and therefore, no simple collection of generalized substituent effects will provide accurate predictions in all situations. Nonetheless, with appropriate spectral data for model compounds available, the total shift effect of several, noninteracting substituents can be predicted to be the sum of the Δδ effects for each individual substituent.

Bicyclo[3.2.2]nonanes

The problem of nomenclature is avoided here because, as a result of the scarcity of data, all data can be displayed on three-dimensional representations. Except in obvious cases, conformational biases should not be assumed from the drawings.

Fundamental Effects on Carbon Shifts

The power of carbon NMR spectroscopy for the solution of structural problems in organic chemistry can be attributed mainly to a single phenomenon — that, in most cases, the total effect on chemical shifts due to the presence of a large number and variety of substituents can be predicted to be the sum of the effects of the individual groups. It is this “substituent additivity” that sets carbon spectroscopy distinctly apart from proton spectroscopy. For example, the chemical shift values for the indicated carbons in ethane, propane, butane, and pentane progress in the order: 6.5, 16.3, 24.8, 34.2 δ[l].