Here is the solution.....
Use HTTP API instead of Native API to generate the access Tokens (Which we can use for authorization)
Follow the below link to Generate OAuth_Tokens
http://oauth.net/core/1.0/#auth_step1
The following are the steps to generate Access tokens
RequestToken : oauth_consumer_key =xxxxxxxxxx(APP Key)&oauth_signature(APP SECRET KEY)=xxxxxxxxxxx
timeStamp = current time in seconds
Nonce = Any random string
https://api.dropbox.com/1/oauth/request_token?oauth_consumer_key= xxxxxxxxxx&oauth_signature_method=PLAINTEXT&oauth_signature= xxxxxxxxxx%26&oauth_timestamp=1388707200&oauth_nonce=jdiwdj39rndkk&oauth_version=1.0
You will get oauth_token & Oauth_Secret now
Now signIn to dropbox account to Authorize the request token
Authorize: https://www.dropbox.com/1/oauth/authorize?oauth_token=xxxxxxxxx
Now you will get the Access token
AccessToken :
https://www.dropbox.com/1/oauth/access_token?oauth_consumer_key=xxxxxxxx&oauth_token= xxxxxxxx&oauth_signature_method=PLAINTEXT&oauth_signature= xxxxxxxx%26 xxxxxxxx&oauth_timestamp=1388707400&oauth_nonce=dji430srgsmx33448&oauth_version=1.0
reply
oauth_token_secret = xxxxxxxx & oauth_token = xxxxxxxx & uid=124926213
Now you have access tokens to access your dropbox with out using user authentications