The armed–disarmed concept

The armed–disarmed concept refers to the relative ease
or difficulty of activating a sugar as a glycosyl donor in
glycosylation reactions.1 Disarmed glycosyl donors have
highly electron withdrawing protecting groups (e.g., esters,
amides) that destabilize the formation of the oxycarbenium
ion/ion pair2 during the course of the glycosylation,
whereas less electron withdrawing protecting groups (e.g.,
ethers) are less destabilizing and therefore arm the glycosyl
donor.

Tetrahedron Letters 49 (2008) 2546–2551

preparation of [Me2SSMe+][BF4-] (DMTSF)

what DMTSF can do?
electrophilic sulfenylation reagent capable of reacting with nucleophilic atoms;² reacts with electron-rich alkenes to promote addition reactions,³ cyclizations;⁴ activates dithioacetals,⁵ trithioorthoesters,⁶ and thioglycosides⁷ for carbon–carbon or carbon–heteroatom bond forming reactions). from e-EROS Encyclopedia of Reagents for Organic Synthesis.

Inorganic Chemistry, Vol. 42, No. 8, 2003
Following the published procedure,
solution containing 0.74 mL of methyl disulfide (8.06 mmol) in
7 mL of CH3CN was added dropwise to an equimolar amount of
Me3O+BF4- (1.04 g, 8.06 mmol) dissolved in 8 mL of CH3CN
at 0 °C. After the mixture had been stirred for 2 h at 0 °C, dry
ether was added to precipitate dimethylthiomethylsulfonium fluoroborate
([Me2SSMe+][BF4-]) as a white solid that was stored in
the glovebox at -36 °C (1.1 g; yield, 69%).

An example of protection of alkene by Diels-alder rxn

New approaches for the synthesis of erythrinan alkaloids

Org. Biomol. Chem., 2009, 7, 1963–1979

retro d-a reaction happened by heating in vacuum. 80% yield.

A synthetic proposal of calicheamicinone

This synthetic plan features a rhodium mediated cyclopropanation and a curtius rearrangement. the overall steps is about 16 which is almost half of the existing synthesis!
The biggest problem I can forsee is the stability of the enediyne before the cyclopropanation.

keyword: calicheamicin, calicheamicinone, retrosynthetic plan, proposal, synthesis

solvents for developing of TLC plates

1. The expected elution order of organic classes

alkanes
alkenes
ethers
halogenated hydrocarbons
aromatic hydrocarbons
aldehydes and ketones
esters
alcohols
amines
carboxylic acids

2. Eluting power of organic solvents

alkanes (hexanes, petroleum ether)
toluene
halogenated hydrocarbons (methylene chloride)
diethyl ether
ethyl acetate
acetone
alcohols
acetic acid

3. some useful combinations:

EtOAc in hexanes
MTBE in petroleum ether
EtOAc in benzene
MeOH in methylene chloride
acetone in methylene chloride

chiral building blocks - Others