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Individual Mn12 single-molecule magnets have been investigated by means of scanning tunneling spectroscopy at room temperature. Current-voltage characteristics of a Mn12 derivative are studied in detail and compared with simulations. A few-parameter scalar model for ballistic current flow through a single energy level is sufficient to describe the main features observed in scanning tunneling spectra of individual Mn12 molecules and offers a deeper insight into the electronic transport properties of this class of single-molecule magnets. In addition, distance-voltage spectroscopy performed on individual Mn12 molecules reveals a possibility to identify the orientation of the molecular easy axis. The results indicate a preferential orientation of the easy axis of the molecules nearly perpendicular to the surface.

X-Ray-diffraction analysis of tBu2CNSCl (4b) revealed an almost linear CNS unit with an SN bond order of ca. 1.9 (Fig. 1), in agreement with the structure of a 1-thia-2-azoniaallene chloride. With SCl2 and SbCl5, compound 4b was transformed into the imidosulfurous dichloride 6 (Scheme 2). With morpholine, compounds 4b and 6 afforded the sulfenamide 7and the aminosulfonium salt 8, respectively. The (diarylmethylene)amidosulfenyl chlorides 4g,h,i reacted with SbCl5 to give SbCl salts of the 1,2-benzisothiazoles 9a,b,d, most likely via 1-thia-2-azoniaallene intermediates 2 (Scheme 3).

The indene derivative 7 and the 1,2,2,3-tetrasubstituted cyclopentane derivative 8 have been synthesized in three steps starting from the Hajos-Wiechert ketone 1.

α-Alkylation of the α,β-unsaturated ketone 1 at C-4 has been achieved by thioalkylation of the corresponding zinc dienolate with α-chlorosulfides of type 2. The desulfurization of the β-(phenylthio) ketones 4 leads directly to the 4-substituted hexahydroinden-5-ones 5.

A total synthesis of (±)-chokol A (rac-12) was accomplished in five steps by starting from the α,β-unsaturated ester (E)-2 in an overall yield of 24%. The key step of this synthesis is the tandem conjugate addition/Dieckmann cyclization of the cuprate derived from vinyl bromide 9 with the α,β-unsaturated ester (E)-2.

The addition of (trimethylsilyl)propargylmagnesium bromide (9) to aldehydes afforded predominantly the allenylic alcohols rac-10. The regioselectivity of this addition changes dramatically when organocerium reagents were used for the addition. In this case nearly exclusive formation of the homopropargylic alcohols rac-8 was observed. Highest selectivity for this process was obtained when cerium(III) isopropoxide (13) was used as a precursor for the formation of the organocerium reagent.

An α-alkylidation of cyclopentanone was achieved in two reaction steps in an overall yield of 20 to 79% via a zinc chloride-catalyzed thioalkylation of the zinc enolate of methyl 2-oxocyclopentanecarboxylate (3) and subsequent saponification/desulfurization of the β-(phenylthio) ketone intermediates 5. For the saponificatioddesuiurization step DABCO proved to be the reagent of choice and superior to the use of alkali or magnesium halides.

The X-ray structure of (+)-13-tert-butoxy-3aa,43,5,6,-7,7a-hexahydro-7a3-methyl-5-oxo-4a-indancarboxylic acid methyl ester is reported, in which the six-membered ring adopts a pseudo chair conformation and the five-membered ring an envelope conformation.

An α-alkylation of the 2-methylcycloalkanones 1 and 4 at the higher substituted carbon can be achieved by thioalkylation of the corresponding zinc enolates with the α-chlorosulfides 3. The desulfurization can be carried out with either Raney nickel or lithium in diethylamine for compounds which contain double or triple bonds.

The lithium or potassium derivative of diethyl isocyanometh-ylphosphonate (1) reacts with epoxides in the presence of boron trifluoride-diethyl ether to provide the diethyl 3-hy-droxy-1-isocyanoalkylphosphonates rac-3. The corresponding mesylates rac-4 undergo a base-mediated cyclization to the diethyl 1-isocyanocyclopropylphosphonates rac-5. Acidic hydrolysis affords the diethyl l-aminocyanocyclopropylphos phonates roc-6, which upon treatment with trimethylsilyl iodide yield the 1-aminocyclopropylphosphonic acids rac-7. A base-mediated cycloalkylation of 1 with dibromoalkanes 10 gives the diethyl 1-isocyanocycloalkylphosphonates 11 which are suitable precursors for the achiral l-aminocycloal-kylphosphonic acids.

The diketopiperazine cyclo(-L-Val-Gly-) (1) can be 0-alkylated under mild acidic reaction conditions with the alkyl trichloroacetimidates 2 to the corresponding bisalkyl bislactim ethers. Alkylation of the bisbenzyl bislactim ether anion of cyclo(-L-Val-Gly-) 4 c with benzyl bromide and subsequent hydrolysis with aqueous trifluoroacetic acid afforded D-phenyl-alanine benzyl ester (8) with an enantiomeric excess of >95%.

Enantiomerically pure 3- and 4-substituted phosphonic acid derivatives 5 of pyroglutamic acid were prepared by a Michael addition of the lithiated (+)-camphor derivative 2 to α,β-unsaturated esters and subsequent hydrolysis. The reduction of the lactams 5 with LiBH4/BF3·OEtz afforded substituted phosphonic acid analogues 7 of proline.

The substituted 2-benzazepines 4, 5 and 12 were prepared by a base-mediated ring expansion reaction of mesylates la- e, which were synthesized in 4 to 5 steps by starting from benzolactams 6.

allo-Coronamic acid (1) was synthesized in five steps enantiomerically and diastereomerically virtually pure by starting from the bislactim ethers of cyclo(-L-Val-Gly-) (3a) or cyclo- (-L-tert-Leu-Gly-) (3b) in an overall yield of 31%. The key step of this synthesis is the intramolecular alkylation of the lithium azaenolate derived from the allylic chloride 4.

A novel method for the synthesis of N-methylserines has been developed. The oxazolidines 4 are formed by condensation of the serine methyl esters 1 with formaldehyde. These intermediates are subsequently reduced to the N-methylserine methyl esters 9 by an acid-catalyzed ring opening via their iminium ions 2 with triethylsilane/trifluoroacetic acid.

The cyclopropanation of the bislactim ethers 3 with diiodo-hymethanes and diethylzinc proceeds in good yields and with high diastereoselectivities, affording the cyclopropyl bislactim ethers 6. Protection of the hydroxy group and subsequent hydrolysis furnish virtually enantiomerically and diastereo merically pure methyl (2R,3S)-2-amino-3-cyclopropyl-3-droxyalkanoates 8. The bislactim ethers of type 3 are obtained by a known method diastereomerically pure, starting from the bislactim ether of cycle(-L-Val-Gly-).

The titanium derivative 2 of the bislactim ether 1 of cyclo(-LVal-Gly-) reacts with aldehydes and ketones 3 highly diastereoselectively to give the syn-addition products 4. Upon hydrolysis with diluted trifluoroacetic acid the compounds 4 yield besides methyl L-valinate the (2R,BS)-threo-serinme ethyl esters 5.

Aldehydes and ketones react with the lithiated bislactim ether 3 of cyc[o-(L-Ala-L-Ala) with 81 to > 95% asymmetric induction (d. e. = diastereomeric excess) at C-3; (R) configuration is formed predominantly. - A model concept for the asymmetric induction is proposed. - With aldehydes or unsymmetrical ketones C-7 of the adducts 6 becomes a chiral center, too. (R) configuration is induced here [for the (3R)-isomers] with d. e. 47-73%. Hydrolysis of the addition products 6 (0.25 N HCI, room temperature) gives L-Ala-OCH, and (2R)-2-methylserine methyl esters 7. Both compounds can be separated either at the ester stage by distillation or - if 7 is thermolabile - after further hydrolysis at the amino acid stage.

The lithiated bis(1actim) ether 8a furnishes with aldehydes and ketones in good yields the addition products 11 with (3R) configuration. The asymmetric inductions at C-3 of 11 (d.e. values) amount to more than 95% with ketones, with aldehydes they are somewhat smaller. With unsymmetrical ketones or aldehydes C-3' also becomes a chiral center. For the (3R)-major diastereomers the "second induction" at C-3' varies from about 4 to about 87% (for benzaldehyde or isobutyraldehyde, respectively), preferably the (3R,3'S) epimers are formed. - Acid hydrolysis of 11 yields (besides methyl L-valinate) the (2R)-serine methyl esters 26. Their e.e. values correspond with the d.e. values of 11. - Dehydratation of 11 furnishes the "Hofmann olefins" 32 and/or the "Saytzeff olefins" 33 which can be transformed in various ways into optically active amino acids.

Enantioselective syntheses of α-alkenyl glycines of type 10 and of type 23 are described that provide these uncommon ammo acids with predictable conf&uration and with ec-values of >95%. Both approaches are based on the bislactim ether method developed by SchBllkopf et al. As for 10: The lithiated his-lactim ether 6 of cyclo(t-val-gly) is reacted with 2-[(dimethyl t-butyl)silyl]alkanals 2 to give the addition products 7 with de> 95%. These on acid hydrolysis afford L-valinate 8 and the methyl (2R)-2-amino4(dimethyl t-butyl)silyl-3-hydroxyalkanoates 9 which are convertible into the (R)-α-alkenyl glycines of type 10. The scope of this synthesis is limited by the fact that the compounds 9 are thermolabile when disubstitued at C-4. As for 23: The lithiated bis-lactim ether 6 is reacted with thioketones 14 to give the addition products 15 with de>95%. The S-methyl compounds 16 undergo elimination to give regioselectively the olefins 18 when treated with Raney-Ni. Alternatively, the olefms 18 are obtained from the sulfonium salts 24 by dimethyl sulfide elimination, although this route is less regiospecific. The compounds 18 are cleaved by dilute hydrochloric acid, liberating L-valinate 8 and (R)-α-alkenyl glycine methvl esters 21. which on further hvdrolvsis vield (R)-α-alkenyl glycines 23. This synthesis is limited only by the availability of thioketones 14.