These chemical reactions and structure elucidations of a few sesquiterpenes - sesquiterpene lactones from Lactarius necator were a part of my Ph.D. work some years ago. As it was the most creative period of my life, I have done this page to prevent forgetting it.
You can find more details in the original articles published in Polish Journal of Chemistry.
Constituents of higher fungi, part XII
Rearrangement of lactone ring of 3-deoxy-6β,7β-dihydro-8-epilactarorufin A
Corrected structures of lactarorufin N and 3-deoxylatarorufin A
Lactarorufin N, a sesquiterpene from Lactarius necator - the corrected structure
3-deoxy-3-epilactarorufin A, a sesquiterpene from Lactarius necator - the corrected structure
This is the abstract of the publication:
Daniewski W. M., Król J., Polish J. Chem., 55, 1247 (1981)
We previously (W. M. Daniewski, M. Kocór, J. Król, Roczniki Chem., 51, 1395 (1977)) discovered a chemical correlation of the structures of vellerolactone (1) with lactarorufin N (2) by dehydration of the latter:
Göran Magnusson and Svante Thorén isolated vellerolactone (1) from Lactarius vellereus and elucidated its stereochemistry on C-3 erroneously - the methyl group on C-3 "down".
They corrected this error. This correction prompted us to revise the structures of the two sesquiterpenes: lactarorufin N (2) and
3-deoxy-3-epilactarorufin A (3)
regarding the stereochemistry on C-3. We made a chemical correlation of the structures of
3-deoxy-3-epilactarorufin A (3)
and lactarorufin N (2) by passing from the latter to the sesquiterpene lactone (3):
Now the stereochemistry of anhydrolactarorufin A acetate (4) (see W. M. Daniewski, M. Kocór, J. Król, Roczniki Chem., 51, 1395 (1977)) could be rationalized:
Both acetyl derivatives, sesquiterpenes (5) and (6) were hydrolyzed:
But neither was the product (5a) identical with 3-deoxy-3-epilactarorufin A (3), the natural sesquiterpene isolated from Lactarius necator, nor was the second one, compound (6a) identical with the second product of the hydrogenation of lactarorufin N (2).
From an examinating of Dreiding models of anhydrolactarorufin A acetate (4) it was obvious that hydrogenation took place from the more accessible convex side, and, therefore, that the hydrogenation products (5, 5a, 6, 6a) have the methyl group on C-3 anti to the H atom of the ring junction.
Examination of the Dreiding models of 3-deoxy-3-epilactarorufin A (3) revealed that it exised in two conformations: "A" and "B", which are shown below:
In the conformation "A" there is a strong repulsion between the hydrohyl group and the methyl group. No such repulsion exists in the conformation "B: and, therefore, 3-deoxy-3-epilactarorufin A (3) attires it. This is confirmed by the value of J H-9, H-8 coupling constant equal to 12 Hz, which corresponds to the dihedral angle 165° for conformation "B".
On the other hand, the examination of the Dreiding models of the synthetic, partially hydrogenated model compound (5a) revealed that the (5a) existed in the two conformations "A" and "B", which are shown below:
These conformations, "A" and "B" are equally favorable, there is no repulsion between the hydroxyl group and the methyl group. This is confirmed by the value of J H-9, H-8 coupling constant equal to 5-8 Hz, which corresponds to the dihedral angle smaller than in (3) and, therefore, we suggest the conformation "A" for the synthetic sesquiterpene (5a).
The dehydration of the synthetic sesquiterpene (6a) with thionyl chloride in pyridine failed. This indicated the lack of trans-diaxial positions of the OH group and hydrogen. Therefore, compound (6a) was oxidized to its keto derivative (7):
The ketone (7) underwent a stereospecific reduction with sodium borhydride, and yielded compound (8) and the rearranged lactone (9):
Compound (8) was proven to be the epimer of (6a) and underwent an easy dehydration with thionyl chloride in pyridine.
The stereochemistry of the rearranged lactone (9) is shown below:
